I know some teachers... and what you're missing is the ineffectiveness and flat-out corruption of the teachers' unions, to whom a large amount of teachers' salary goes toward.
Look at Chicago, but that is hardly the only example.
The unions are doing nothing about poor pay, respect, ventilation, work-life balance, classroom sizes, or anything else. All the unions are doing is collecting their tributes, releasing a political statement every other month, and paying themselves a lifetime annuity.
It would really be helpful if there was a group that actually had teachers best interests in heart.