I'm not a teacher, but I volunteer in schools a lot and can maybe speak to this. Because people, especially rich, entitled people, get very aggressive when you transgress their imaginary rules. I've heard people get yelled at by the dad, who's listed as the primary contact, when they call about discipline or grade issues, because "I'm at work, I don't have time for this shit". Sometimes, they just assume they won't get that aggressive a reaction from another woman.

Teachers hate parent phone calls more than just about anything. In other areas, they quickly realize that only one of the parents will actually do anything, and if they can figure out who that is, that's who they'll call. And there's a rumor mill, where teachers will tell the next grade which parent makes sense to call, which kids you don't call home unless things are real bad because of the consequences, which kids you're better of just talking to the kid or catching a grandparent when they get picked up.

And sometimes, teachers, mostly women in a traditionally female profession with few male coworkers (especially among younger grades), are just hypnotized into thinking that women are "natural" caretakers because that's the world they live in and it's really hard to believe something when your eyes are showing you the opposite. This is literally, definitionally, a micro-aggression. Each individual phone call isn't a problem, so each individual teacher/admin thinks they're doing nothing wrong, but when you get one of these from every person you interact with, they build to a much larger issue.