Its because the state has a near monopoly on hiring teachers, so they can keep wages artificially low.
Let's double the teachers' wages, but first you need to double the budget, and for that, need to first double the local property taxes.
Put doubling the property taxes on the next ballot and see if that passes. If not, there is no money to raise the wages for teachers.
Where wages can go up is when teachers move from a cost center to a profit center ... such as with a private school, though that is still limited by the amount that people are willing to pay to go there.
A local private high school (midwest - median household income is $75,000) is $16,000 per year with enrollment of about 500 students. You have $8M to use. There are 100 people on staff (this includes grounds and maintenance). Let's apply a broad stroke and say that this is 75 teachers and 25 kitchen, maintenance, grounds, and secretarial jobs. Pretending that all other expenses are $0 and that the non teaching staff is $0 also... $8M into 75 is $106k. That places an upper bound on the amount that teachers make there ... unless we raise that $16,000 tuition or find other revenue sources.