Because Democrats in the US are right-wing too. And they know as long as Republicans keep pushing literal comic book villains, leftists are forced to vote for them. They don’t have to try harder, because they know any leftist that actually knows how the system works will vote for them even if they just do the bare minimum.

The only way for that to change is to keep voting for the lesser evil, because once Republicans lose enough terms in a row they’ll have to shift more towards the center, and at that point Dems will be forced to actually go left because “we’re not threatening democracy” won’t be enough anymore to get elected.