That Biden anomaly is fascinating.
Based on PPs' comments, I assume enough people who sit in the middle (who feel unrepresented by the US choices in politics) felt sufficiently mobilised to vote against the direction in which Trump had gone during his presidency i.e. they voted for change rather than for Biden.
If I lived in the US, I certainly wouldn't feel represented by the available choices. I'd be someone perpetually stuck in the middle. Each choice is an "all in" bucket, each encompassing values at its respective core with which I wholly disagree. For example I am strongly anti-guns + strongly pro-choice on abortion and I am strongly against the authoritarian enforcement of gender identity as fact in education, healthcare, sports etc.
I believe that the economy and most other things that impact daily life benefit from a swing between left and right values every few terms of government. But in the UK, our choices are normally sufficiently all in the middle. Normally, I can stick with my Lib Dems/left vote every time I vote (because they encompass my values best) and be confident that we'll have mostly sensible political decisions no matter who gets in. Although this year, after about a year of planning to spoil my ballot (because we had the same authoritarian issues in all the parties on the left) I finally voted for my Tory candidate, despite how pissed off I was at the Tories at a national level. My dislike of the gender identity authoritarianism combined with my candidate's strong record on local issues was enough to balance out my anger at the Tories for partygate and Boris' gaslighting over Brexit. It certainly helped that Boris had gone.
So if I lived with the US options as they stand right now, I'm guessing that every now and again, I'd have to vote for the opposite of now (vote for a change) but I'm still not convinced that I could do that with Trump as the Republican leader. If spoiling my ballot wasn't an option because of the voting system, I think I'd have stayed home this time to voice my dissatisfaction at the options.
If the Democrats remain wedded to identity politics as a core value in addition to pro-choice on abortion (plus other traditional left values), I'm guessing it'll take another couple of terms of Trump before the missing middle-ground protest voters get sufficiently mobilised again to bump him out.