The reason is the "make-work" rules of the "edukachun" unions. Until we break the backs of these leeches, the "publik skools" will continue to decline in quality and increase in cost.
I have had at least 12 excellent -- meaning "life-changing" -- teachers in 22 years of education, and counting. Less than half of them were certified, public school teachers, and two were not even "teachers" at all, they were a librarian and a schoutmaster.
Do the best for your children, which probably means keeping them away from the public schools. But do the best for your community as well. If you have an elected school board, consider running for it. I have not heard of a home-schooling parent being elected to a school board. It could be an eye-opening experience for the Board, for you, and for your community.
Congressman Billybob