Easy answer: because they are led by centrists who stand for nothing much beyond being in the middle so something can get done. The very nature of centrism is to not be in favor of either side of a political spectrum. Everything is negotiable, lately even the current value of human life abroad, which will come back to our shores. Centrists sure as hell cant lead much of anywhere coherent, or win elections. They can be proud of something like an infrsatructure bill, and they feel like that should be enough to get voters to the polls for them. Its just not. Anything uncontroversial enough that the right wont object to is not going to animate the left either. They can’t win on road-building and post office branch naming.

Parties need money to exist. Without voter funding from people and groups who want to get specific things done, the only funding they can get is from corporations who want dirty favors, so centrists inevitably become the dirty corporate favor party too. Or, “whores” if you will. Its whore or starve for these folks.