Many (most?) Americans don't seem to have ever learned how to think critically or question what authority figures tell them to believe.
On top of that, COVID was rough for lots and lots of people. Even though it started under Trump, he somehow managed to avoid blame for the government's missteps early on. Biden did what he could, but even an absolute perfect response would have caused a lot of strife for a lot of people, and his administration's response was definitely not perfect. In a way I think it's impressive how well Harris did; even had Biden decided not to run at all for a second term, it would have been an uphill battle for the incumbent party to stay in power after COVID.