(Servo is arguably in the middle, accepting outside contributions as long as you don't use AI.)
It's understandable that a team without much funding would have to close off contributions to spare on labor costs. But, it makes me feel that people don't give Google/Mozilla/Apple enough credit for the economic resources they put into enabling openness.
(Personal bias/experience alert: I'm currently retired, but formerly worked at Google on Chrome. I saw many of my coworkers nurture outside contributors, and did some of that myself, both informally and through programs like internships.)