The thing to remember is that Trump is a character, and that character speaks a lot more to people than Biden does. A lot of US voters care more about the person in power than the policies they and their party support - which is dumb, but when all you can think about and hear from is one person talking about what they talk about, that's what a lot of the focus is on, as opposed to what policies their party supports and are in support of.
Then the next thing is Trump says a lot of shit about minorities - but that's the whole point, it's the minorities that are affected, not the larger voting populace. If you're not part of that minority, it's /really/ hard to put yourself in their shoes and realise how bad it is for them if Trump gets elected. Think about BLM (CW: racism and what led to the BLM movement)
CW: racism and what led to the BLM movement
If you weren't in the minority that suffers from police violence, all you saw was George Floyd being killed by a group of police officers. That was it. When you've spent your whole life being told the police are good, the police will protect you, and have had no evidence until now telling you anything otherwise, you're seeing people say a group of people you believed were beyond reproach as the literal worst thing in existence. That dissonance won't make you question the police, a group you love - it'll make you question Floyd, and his character, the thing you don't know ANYTHING about.
For every black person in America, they saw that, and they saw all the problems they've had to deal with come to light. It seems insane to think this, but African-Americans only represent 13.4% of the American population. That's /tiny/.
And it's a similar story for all the other horrible things Trump has said - they affect minorities, not the main voter demographic. Combine gerrymandering to control votes such that these minorities are split far and a part so that they have little affect on the vote, and you have a recipe for them to be drowned out and forgotten. More than 75% of Americans identify as only white, based on census data from last year. Easily enough to drown out the minority, and requires more than third of those white people to think outside their own circumstances and take a stand to help support the minorities.
There is one final thing I'll say - my housemate said it a lot, and I think it holds true here. In times of crisis, you stick with what you know. I think that's going to reflect a lot here. Sure, Biden might improve the response to COVID and save lives - but it's equally likely what he does will have no affect, and maybe it'll ruin the economy. At least with Trump, they know what to expect, and can just keep doing what they've been doing to fight it. I don't think this is a /good/ attitude to have, but it would explain what we're seeing in the amount of support Trump still has.