That's a fair question and I think Canuckus provided a pretty good answer. But you know me; I go into depth and you *did* ask why... So you may not like my opinion but I am providing some examples that I think explains the argument in more detail.
In economic terms, we are still the largest consumer market in the world, so we certainly say we lead the world in buying things, but that's about it. We've been falling behind in production since the 80's. Trump supposedly tried to revive manufacturing and there was indeed a rise in jobs during Trump's first two years when the economy was still benefiting from the momentum created under Obama. But in 2019, the first year affected by Trump's economic polices (if that's what you want to call a rash of tariff taxes and a tax cut for the wealthy) the growth rate actually started to decline and in January of this year, BEFORE the pandemic hit, we saw manufacturing take a dive.
Trump fans will point out the isolated examples of jobs created here and there through Trump's "deals" with specific companies. But these "stunts" have served more as symbolic references for bragging than an overall indication of revival... The realists aren't fooled. In the big picture, there was a net loss of jobs under Trump, and the world knows it.
In diplomatic terms, we did alright until Trump came and made it crystal clear to the world that we were not going to honor our promises. There is no better way to concede your leadership than to demonstrate the worthlessness of your promises.
In terms of leading by example, the world is very aware that the U.S. has the lowest rated school systems, the worst (AND most expensive) healthcare systems, the highest incarceration rates (we pay more for prisons than we do for schools) and the most exploited citizens in the developed world. We also have one of the most dysfunctional democracies. If that isn't enough to look elsewhere for inspiration there is also the fact that Trump supporters are giving the rest of the world the impression that as a population, we are not very bright.
In terms of military leadership, we are a far cry from the inspirational power that we were hailed for being at the close of WW2. We DO spend more money on our military than the rest of the world combined and our technology is second to none. But although this does result in a very scary looking military force. There is a reason why we are developing an "all bark, no bite" reputation.
We failed to defeat North Korea, we lost the war in Vietnam and although we were on the winning side in the Balkans, we only played a limited role where all the Europeans wanted from us was our expensive airstrikes because that's basically the one thing we do well.
The Gulf War was probably the last moment where the U.S. was seen as a viable military leader. We might not have been as successful without the help of Saudi Arabia, Egypt, Britain AND France but George W.H. Bush did lead the coalition and the war was well deserved since Iraq did invade Kuwait.
Bush Jr, set that reputation way back when with great fanfare he invaded Iraq, a country that was already defeated, stripped of all it's defenses (except the Republican Guard), squeezed between no-fly zones and monitored by U.N. weapons inspectors. His invasion was based on the false claim that Iraq had WMD that posed an immediate threat to the U.S. Of course such weapons were never found. This time around, the French, the Saudis and Egypt declined I'm guessing because they didn't want to be involved in kicking the crap out of a defenseless country for the sake of controlling their oil.
Then the world watched the results... In Iraq, the rise of the Sunni insurgency, which eventually developed into the ISIS caliphate. In America, the largest economic crash since 1929 and a commitment to a no-exit state of war in the middle-east.
Hardly a model for inspiration.
Obama helped fix the economy and cranked up the engines that pushed it forward all the way to 2018, but he hobbled through the wars in the Middle-East. Nevertheless, he didn't do anything to compromise our position as a military leader.
But then came Trump and all of a sudden, America was OK with being the asshole in the global community. Trump likes to take bows for destroying ISIS but Iran had far more to do with that under Soleimani since they did most of the fighting against ISIS. But Trump had Solemani assassinated and tried to take credit for destroying ISIS. What a dick.
Trump supporters, in their personal silos of "reality" will swear up and down that Trump deserves the credit and that Soleimani killed Americans, not ISIS fighters but the world knows better. And no one wants to be led by someone who killed the general that defeated the enemy then takes credit for his success.
Canuckus also mentioned the election meddling by foreign powers, but this is the one point he made that I'm not 100% on board with and only because Russia AND China are meddling with elections all over the world, including the last election in France. I wouldn't be surprised to find out we're doing the same thing. In simple terms, the government doesn't really have a way to control social media without serious infractions on free speech. I'm not even sure how much we can blame on Moscow or Beijing because a lot of people, especially in Russia and Eastern Europe are finding ad-based commercial success in routing popular content through their websites and right-wing conspiracy theories just happen to be ranked #1 in terms of popularity.
I wouldn't even suggest Trump had as much to do with the meddling in 2016 as the young Russians and East East Europeans running websites in their basements. But it's hard to imagine he didn't benefit from it.