Apparently your knowledge of history begins in 1787 or thereabouts and is confined to the fifty United States. There is nothing absurd about my premise. For most of history slavery was considered a normal way of doing things. The divine right of kings was considered moral. The Catholic Church burned inconvenient people at the stake and stole their wealth and it was considered moral. Muslims brought fire and sword to half the known world and it was considered moral by them, and moral to kill them by their opponents. Slaves were chattel through most of history. One hundred fifty years ago, public floggings and hangings were considered moral and proper.
One more time. Morals are individual and societal survival traits. In the harsher world of times past, harsher things were considered right and proper. Whether you like it or not, when you sit on top of the molehill of obstacles you face compared to the summit of the mountain of problems that people of times past had to contend with. It must be a hard thing to realize that your own myopic worldview of what is moral is not even always relevant to today, much less to hundreds or thousands of years ago.
Morals are malleable. Morality is determined by physical reality. What you consider moral today would be considered lunacy by people a thousand years ago.

Your description of morality is your own poorly though through opinion. Your thought patterns are in a rutted road without much traffic. You have your safe, secure little world in which you are always right and you will not venture out from it. Historical reality, to you, is an abstract concept rather than something that actually happened that helped impel the events of today. Physical events transpire because of prevalent thought patterns. Moral behavior is that which promotes, or is thought to promote the survival and well-being of the individual and group. When the morality of two groups collide, the moral precepts tend to follow the winners.

Your definition of morality is behavior that "promotes, or is thought to promote the survival and well-being of the individual and group." THE definition of morality is "principles concerning the distinction between right and wrong or good and bad behavior." Recreate the definitions of any words you like to make your snowman case to promote or defend, something you can argue with all your "alternative facts" to make a point. Impossible to have a conversation with a person where all facts and truth are decided on their whim for their own end.

This is the site: https://www.newyorker.com/magazine/2013/12/09/our-broken-constitution

Very interesting and intriguing and deep dive into the currents of Democracy. It is long, somewhat tedious, and erudite. Probably not to the taste of many here, who seem to prefer 10 second sound bites. Yet it will surprise and disappoint many more with its revelations about the pronounced failures inherent in what most see as a venerable and miraculous, perhaps God-inspired, document. Sacred. In reading this piece, I am of the opinion it is not anything like that. Every time it has been tried in other countries, it has failed miserably and led to oppressive governments. Its longest lifespan was about ten years but the average was five. How did it appear to work here for so long? It really didn't.

Here is an excerpt to excite or dull your taste buds: "In creating the national legislature, the delegates had to address the issue of slavery. Although slaves weren’t citizens and couldn’t vote, the Southern states wanted them to be included in the calculation of the over-all population, in order to boost the region’s representation in the House. The North thought that the slaves should not count at all.

"In a way, the negotiated solution reflected the shameful reality that slaves in the United States were judged less than fully human. The standoff led to a notorious compromise: for purposes of apportioning seats in the House, each slave would count as three-fifths of a person. As the University of Pennsylvania historian Richard Beeman noted, in 'Plain, Honest Men,' his 2009 account of the Convention, the debate over the three-fifths rule took place with 'a near-total absence of anything resembling a moral dimension.'”

This is the site: https://www.newyorker.com/magazine/2013/12/09/our-broken-constitution

Very interesting and intriguing and deep dive into the currents of Democracy. It is long, somewhat tedious, and erudite. Probably not to the taste of many here, who seem to prefer 10 second sound bites. Yet it will surprise and disappoint many more with its revelations about the pronounced failures inherent in what most see as a venerable and miraculous, perhaps God-inspired, document. Sacred. In reading this piece, I am of the opinion it is not anything like that. Every time it has been tried in other countries, it has failed miserably and led to oppressive governments. Its longest lifespan was about ten years but the average was five. How did it appear to work here for so long? It really didn't.

Here is an excerpt to excite or dull your taste buds: "In creating the national legislature, the delegates had to address the issue of slavery. Although slaves weren’t citizens and couldn’t vote, the Southern states wanted them to be included in the calculation of the over-all population, in order to boost the region’s representation in the House. The North thought that the slaves should not count at all.

"In a way, the negotiated solution reflected the shameful reality that slaves in the United States were judged less than fully human. The standoff led to a notorious compromise: for purposes of apportioning seats in the House, each slave would count as three-fifths of a person. As the University of Pennsylvania historian Richard Beeman noted, in 'Plain, Honest Men,' his 2009 account of the Convention, the debate over the three-fifths rule took place with 'a near-total absence of anything resembling a moral dimension.'”

The Constitution isn’t broken, we are............though not as badly as we were 250 years ago. The Constitution’s purpose has always been to provide the vision of a better society, peopled by better citizens. Had the Declaration and Constitution not been written the way they were, there would not have been a legal foundation for abolition. Slavery was always an abomination and was the currency of global trade for centuries. It took the combination of Western Christianity and representative government to finally abolish it.

This differs from the way slaves had been perceived for thousands of years HOW?
Like most Liberals, you judge the morals of the people of a bygone era by modern Liberal standards. Morality is malleable, and is dictated by exigent circumstances.
Slavery was considered a normal practice for most of the history of humanity, yet Liberals seem to think that it began and ended in 18th and 19th century North America.
Liberals constantly gripe about the evils of slavery in this country while absolutely ignoring the FACT that the practice continued legally in Africa until the 21st Century, and still is practiced world wide.
Amnesty International estimates there are currently about 20 million people held in some sort of involuntary servitude in the world, yet rather than address this very real problem, Liberals blame white Europeans for all of the attendant evils, even though it is mostly practiced today in Africa and the Middle East. It doesn't count there because evil white Americans are not responsible for it, apparently.
Liberals, with their parochial, myopic and self-serving, self-righteous talking points, ignore any wrongdoing that is not wrought by white Americans.

3/5 of a person, not 2/3. And 2/3 is far, far better than the South would allow. Just more well-informed, sorry.

How did they get to 3/5 during the 1787 United States Constitutional Convention? This population number would then be used to determine the number of seats that the state would have in the United States House of Representatives for the next ten years. The compromise solution was to count three out of every five slaves as a person for this purpose. Its effect was to give the southern states a third more seats in Congress and a third more electoral votes than if slaves had been ignored, but fewer than if slaves and free people had been counted equally. No matter what our Declaration of Independence lied about being "created equal."

3/5 of a person, not 2/3. And 2/3 is far, far better than the South would allow. Just more well-informed, sorry.

How did they get to 3/5 during the 1787 United States Constitutional Convention? This population number would then be used to determine the number of seats that the state would have in the United States House of Representatives for the next ten years. The compromise solution was to count three out of every five slaves as a person for this purpose. Its effect was to give the southern states a third more seats in Congress and a third more electoral votes than if slaves had been ignored, but fewer than if slaves and free people had been counted equally. No matter what our Declaration of Independence lied about being "created equal."