Because Trump wants to overturn the Constitution in order to keep himself in power. because Trump incites his base to violence (like right when he threatened "death and destruction" if he is arrested). Because Trump has already left a legacy in the Republican party of denying election results any time you lose, and has laid the groundwork for undermining democracy.
Don't get me wrong, I agree that he has no actual moral convictions, he's just trying to get and maintain power. Desantis is dangerous, 100%. But Trump is a destabilizing force and I don't know if American can withstand another 4 years. I'm not even convinced that the first 4 years wasn't the gut wound that slowly kills us.
The DNC is awful, too. No way they let anyone in who would actually be good for us.
Basically, we're fucked.