Congress pretty much could now -- the Constitution is actually quite vague as to what constitutes a "crime or misdemeanor" sufficient for employment. But even then, there're two considerations. One, it'd better be very clear-cut, and Trump still has enough support that things could get seriously ugly from a civil unrest perspective. More than that, though, is the fact that (as keifermiller points out), the Democrats have given up all pretense of running on anything other than not being Trump. Having actual policy positions is hard, I guess. Since that's their plan, though, they can't remove Trump because then they remove their entire reason for existing.
That's certainly been part of it, but they at least usually go on to explain the ways (even if made up) that they'll be different.