Doesn’t that have more to do with how hard it is to hire bus drivers?

Yeah, that plus our union makes it very hard to fire people, even for gross illegality. Don't get me wrong, I'm 100% pro-union (we make about $10 more per hour than our neighboring non-union districts and have health insurance and a pension) but there's definitely a downside to it. Our shop steward got a DUI after blowing a fucking .36 (she was found passed out in her car with the engine running) and was driving her route three hours later, and she was eventually allowed to resign rather than being fired. Shop stewards are even more protected than regular union members.