This Reddit thread has some relevant comments that relate to the fact that the significant risks of leaving a job break the most critical feedback loop that could have a real impact on management quality. Too many managers keep their staff because any job is better than no job, not because it's an enjoyable way to spend their time.
I've been lucky in that the one time I had a truly bad boss I was unable to avoid dealing with, I was able to quit cold, rather than finding another position to go to. I wasn't the only one who was having problems with that individual, though, so the end result was that he left rather than me, and we were able to start improving the work environment for everyone.