Trump was the political equivalent of a wartime president. Not a foreign war, a domestic political one. But it’s the same dynamic. People under siege rally around their leader.
Trump was and is an extremely polarizing person. But it wasn’t just him under attack for his personal failings (though that was was part of it too). A good portion of the extreme hostility to Trump was towards his political views and statements which he shared with his voters. So they felt under attack too, and rallied around.
Biden comes in for his share of hostility, but that’s within the bounds of normal politics these days (much like any of the past presidents in the last few decades, of either party). So Biden’s floor of support is lower than Trump’s is.
Biden’s ceiling of support is also higher than Trump’s. IIRC, Biden’s high approval rating was higher than Trump’s ever was - I believe Trump never cracked 50%. But that’s the flip side of the same phenomenon.
One reason Trump believes it’s impossible for him to have lost the election to Biden is that the crowds at his rallies were far larger than those at Biden’s. But that’s because the election was all about Trump, and not all about Biden. Trump had more fervent rally-attending supporters than Biden, but he also had more die hard opponents as well. Just that the latter are less likely to attend any rallies. I saw a quote from a poitical strategist a couple of months before the election which captured it: “this election is Trump versus Trump, and Trump is losing”.
Leaving aside Biden’s actual successes and failures as president, the fact has been - for as long as I’ve been following politics - that presidents get credit or blame for “how things are going” regardless of the extent to which it’s connected to anything they might have done. Most voters, and in particular the low information voters who are likeliest to be swing voters, have a very rudimentary understanding of the complex forces which shape the economy or world politics. They have a vague idea that if a president is doing a good job it will work out and if he’s not then it won’t, and that’s about it.
To some extent, politicians don’t deserve a whole lot of sympathy about this. Because all these people, when running for office - and president in particular - tend to claim that if they’re elected they will solve all the country’s problems. If they’re really limited in what they can do about the country’s problems, then they perhaps shouldn’t say otherwise upfront when they’re trying to get people to vote for them.
OTOH, “everyone is doing it”. If a guy ran for president and said “hey, I’m going to level with you, a lot of the problems we have are intractable, and even to the extent that we make minor headway against those, it’s very possible that other intractable issues will arise”, and some other guy is running saying “if you vote for me, everything will be great”, then that first guy has very very little chance of getting elected. So it’s back to the people again.
It’s as if a used car dealer decided to tell you all the problems that his cars have, while his competition is talking their cars up. That first guy is unlikely to make it, which is why there are few of such guys in business. Politicians are like used car salesmen in that sense. If they luck out and the tide rises while they’re in office, then they go down as great leaders. If the opposite happens, then they go down with the ship.
At any rate, the bottom line is that Biden’s level of support is more malleable than Trump’s was, and with dissatisfaction in the way things are going, he takes a bigger hit.