In-group membership tends to be sticky. People can stay in a tribe even when their qualifications in someone outside of the tribe would be insufficient to let them in.

I imagine that's partially driven by a feeling that one's previous qualifications lend some predictive power that they will earn their place again in the future even if right now they're foundering. Also, I suspect a lot of it is that people don't want to be in groups where it's too easy to get kicked out because it makes themselves feel to tenuous. So the kinds of in-groups that stick around are the ones that are a little more generous to their members once they get in.