Tribalism. And in addition to family, it can be the people you work with and definitely is the people you go to church with. Throw in some confirmation bias and you have scenarios like people easily believing that coach k is actually Satan incarnate, rather than a guy who looks like a rat but actually is great at putting together a team that plays D and puts the ball in the basket without too many fouls being called.Seems like politics these days is a lot like sports fandom. I know many people are fans of a team due to some personal connection (you went to that school or you're from the city where the team is based or you moved there, whatever). Unfortunately there are only two teams in this sport and the other side is the most hated rival. This isn't everybody, to be sure, but I think it's a lot more than anybody wants to admit. It's not about policy or messaging or any of that, it's just you live around lots of people that are fans of one of the teams and so you're a fan of that team. I think this applies to Maga more than dems or traditional repubs. They probably won't even care if Trump's policies cause their team to lose, they'll still be a fan of their team, just like sports fans don't stop being fans of their teams every time they lose, the reactions of some fans on message boards every time their team loses notwithstanding...
I think this accounts for big chunks. Some of these people can swap sides if they don't fall for the misinfo though.