Because the stakes of this election are so very, very high. Trump genuinely and explicitly wants to create a fascist state; it's borderline incomprehensible that somebody would choose to sit out and let that happen just because they don't like Biden.
Just to be entirely clear--are you taking the position that given the choice between (a) the world where you don't vote and Trump wins and (b) the world where you vote Biden and Trump loses, you would take (a)?
Just to be entirely clear-- real people are dying right now because of things Biden could directly control. Trump has nothing to do with that. Biden has a crackerjack team of experts that are obviously doing political calculations that currently don't require my vote for him to win. I wouldn't stress about it if I were you.