Because public education has become a vector for propaganda.
Because we spend more per student but with awful results.
Because our brightest don't become schoolteachers.
Because education is years if not decades behind the skills curve.
Because big, powerful teachers unions make change impossible.
Because parents have spared the rod and spoiled the child.
But also not functionally a union. Teachers are actually represented by their state affiliate, and whether or not they can collectively bargain (the sine qua non of being a union) varies from state to state. Personally, I've lived in several states, but none in which the state Education Association was able to collectively bargain for its members.
Every K-12 teacher I've known (male and female) has had times when they've come home crying from the stress. (workload, the kids you care about and can't help, ...)
The NEA does something about pay and tenure but when it comes to protecting teachers emotionally forget about it. Roughly half of the people who get a teaching certificate at their own expense discover it is a job that they can't stand to do.
I don't think I've ever heard "big powerful unions" and "teachers" in the same sentence before.