Turnover in tech is extremely high and it's not because devs are constantly being mismanaged, it's because they are egotistical and capable of working independently and often just do without any real penalty. "Oh you fired me, well 9 times out of 10 I'll just be earning more money across the street, good luck recruiting anyone with a bad Glassdoor review!" What incentives does the manager have to get invested with the devs in this situation when employees good and bad can just leave easily.
Anyone who's worked in tach a while has seen cases where a dev has just flat out refused to do something a manager has asked them to do. Why? Ego is the only reason when you get paid either way.
Look at all the egotistical devs on this forum alone saying "all managers are bad" without considering they could be part of the problem.