a) I think part of it is the work it creates to orient a new member to the songs. It's boring to have to bring ANOTHER person up to speed.
My original jazz band -- the one where I was constantly posting drama about as I figured out how to manage it properly -- was like that. There was a member I wanted to fire, and everyone acknowledged it was my band and I could do it, but they said they didnt' want to rehearse a new guitar player. I let him go, and had to spend a lot of time orienting new players. And given how musicians are not beholden to any one band, I was always subbing and the work fell on me personally to orient new players.
I learned efficient ways of doing so, and emerged as the person in the end who could pick whoever I wanted for any gig, from a menu of multiple players. I rotate and use players at will.
You want to get to that point if you can. I am doing that now with singers. It's harder with rock bands and singers, though.
That's why i wish the Real Book was produced for rock bands so everyone knows rock standards and can read them on sight like we do in jazz. I'm getting there as in my rock group, they are all jazz players and play off charts -- if only the singer would stop changing the structure on us!!
b) Also, what is a deal breaker for you is just a little irritant to others. Had one guy who would insult my age, clothes, weight, rehearsal room, profession, height, tone of my bass, approach to organizing the band, and on and on and on -- whenever he could. Later he proudly confessed he was a narcissist. This to me was intolerable -- to everyone else? The guy was a jerk but it's hard to find good keyboard players so just put up with it. I eventually quit.