Why doesn't he become an independent like Justin Amash did?
If he becomes a Republican, he is repudiating all the values of the majority who voted for him.
I don't know of a single Democrat who doesn't believe that Trump is incompetent, lazy, personally repugnant, dishonest, corrupt, narcissistic, cruel, and more. There is no way any of the people who voted for him would accept such a change.
It is one thing to become an independent and quite another to embrace the poisonous politics of Trump.
Most Democrats I know are apprehensive, because they know that Republicans in the Senate have refused to pass laws needed to protect our election from foreign influence, and because they know how much dark money will be thrown into the election. They are afraid of Biden being nominated because Trump, as president, has ordered his accomplice in the DOJ to reinvesigate what the Bidens have already been cleared of. They will "pretend" that this sham of an investigation will lead somewhere.
There were people who voted for Trump who knew his character (and lack thereof) but they convinced themselves that "Hillary was worse." Trump knows that if he can construct a false narrative so that some voters can believe that Biden is "as corrupt as Trump" (I don't know what planet they've been visiting...) that they will vote for him.
Hillary had more integrity in her little finger than Trump did in his whole body, but people were duped into believing they had to pick from two corrupt choices.
Proportion, people, proportion!