I get it, Biden is old and that's a problem, but why doesn't anyone seem to have a problem with the fact that Trump is almost the same age, has 34 felonies, raped at least 2 women, went to Epstein's Island 11 times, sexualized his own daughter, stole classified documents, aligned himself with Xi and Putin, and can't remember common names?
Is all of that really better than being 2½ years older than he is right now?
Most people who want him to drop out were never going to vote for him anyway.
People (perhaps erroneously) perceive a chance of Biden actually doing it. Trump will never drop out as winning the presidency is his only shot at avoiding incarceration. He'll probably run again in 2028 if he's still around by then.
Trump isn't incumbent, meaning the GOP already had a full primary in the past year. They've already determined they don't have a better alternative. Technically the Dems could have nominated someone other than Biden, but it was never a very likely possibility, so people aren't as familiar with Democratic alternatives.
Republicans either don't believe any of the criticisms you made of Trump are true, are okay with them, still believe him to be the lesser of two evils, or some combination of these. This is largely due to right-wing propaganda e.g. Fox News, OAN, Info Wars, etc.
I'm no expert, these are just what I perceive to be happening. There's probably other factors at play as well.
Edit: with the recent supreme court rulings, I'm guessing that he may be able to avoid jail even if he doesn't win. I still don't see any chance he'll drop out.
The constitution has decreed a two term limit, now let us see it enforce it. Pieces of paper may tell us how power is to be used, but in the end paper is paper, and power is power. A lot of Americans call on the power of the constitution while ignoring what it says.