Soul-crushing lack of accountability is a factor as well. Outside of physically assaulting someone or stealing a bunch of shit it is almost unheard of for someone to be terminated for either incompetence or negligence unless it's so optically bad for the district or administration as a whole that they have no choice.
Then you have to take into account the skillsets that you're left with when capable people leave. In my experience, those that can swim best often jump ship first and with them take knowledge that was either carelessly preserved or is totally unattainable by the staff that remains. Positions are sometimes never back-filled leaving less capable staff to pick up slack and the cycle continues, things get overlooked and stagnate and smart, bored kids own your ass.