Because the teachers union is too powerful. If you can't fire bad teachers, you can't afford to pay good teachers well.

There is no way to evaluate the effectiveness of teachers except for the results from their students, which more often than not hinge on school budgets, demographics, and the involvement or lack thereof of parents. 

Good teachers don't teach for the money, so raising the salary wouldn't attract more good teachers; it would only attract more half-ass money-grubbing slackers, of which we have enough already.

As a teacher, I would love to earn doctor money for what I do. But I wouldn't want to have to go to ten more years of university first. Teachers are best when they're young and overly optimistic. 

If school districts had that kind of money, I would rather they use it to hire an underling for me to do all my paperwork. Money can't buy time. I suspect more doctors would trade a good bit of their salary in order shorten their work hours.