Because there aren't really any consequences for the democrats... The ones in safe seats keep their jobs which are basically useless for a few years, what a cushy job. There is no moderating force in a two party system, there's no external threat. One party can drive the public to outrage, and then lose the election and everyone chills out only for it to be replaced by an equally dysfunctional and regressive government. Voting Dems is only delaying the inevitable collapse, voting R just accelerates it. America needs to realise that a centuries old political system just isn't cutting it any more. Two party systems are meant to force the sides to converge on the median, so you get two very moderate parties, but now one is moving further and further right and the other seems to being dragged along with it. At least ranked choice voting might be a start.