What seems to be new now, is that Group B has taken to espousing Group A's caricature as a way of proving their group identity. "Group A says we eat babies. I eat more babies than anyone else, proving my Group B bonafides!" If you want to stand out in the primaries, you can't be a competent, coalition-building centrist. You gotta eat more babies than the next guy.
Current political system reinforces extremism. I'm of the camp that believes we urgently need incremental reforms like Ranked Choice Voting and open primaries. It doesn't solve everything, but it's the reform we can do that gives us the most mileage today.