The thing you're glossing over is that a lot of Trump voters weren't really voting "for" Trump so much as against Democrats. This also happens on the other side as well, most people I know that supported Biden only did so because he wasn't Trump.

Democrats have some policy positions that are troubling for a lot of voters. Gun control is a big one. Much of rural America will vote against Democrats every time just on that one issue alone. Also, defunding the police is unpopular with many voters. There are also many who have moral concerns about gender transition surgeries for minors or allowing biological males to compete in women's sports.

Also, the quality of Democrat candidates for 2016 and 2020 was very low.