There are two parts. First, they aren’t as underpaid as most people think in most cases. The union isn’t dumb. When they negotiate they look at the long term. A career teacher (30 to 35 years) can retire at about 55 give or take depending on the district. And they will get something like 80% of thier salary for the rest of thier life. They will also get subsidized health insurance. And in some states, all of that is tax free. That is a ton of money and a ton of security. And for many, they can retire, collect pension, and go get another job at the same time if they want. I make more than double what teachers make best case, and my wife works too for a 6 figure salary. I can’t possibly retire at 55, let alone feel secure doing so. I also have been laid off twice over the last 30 years, where as most teacher don’t have to worry about that after 10 years. Now, I get to take vacation anytime of the year, I can change jobs or move and not mess up my future benefits. I don’t have to deal with parents. Lots of intangible benefits to not being a teacher. But the point is the union ensures those less obvious benefits, which keeps the current salary low. This keeps the optics of drastically underpaid teachers so that the union can still negotiate for more with public sentiment on thier side. So while they are still underpaid, it isn’t as drastic as it would appear.

The other reason is simple. There are a lot of teachers. Like a lot a lot. And schools are generally built to a higher standard of saftey, so they are much more expensive than other building types. All of this adds up to a very high cost. Education is typically one of the largest expenditures for a state budget. Poloticians could dump more money into it, but it isn’t likely to be enough to make a difference that will get them reelected. So they put money other places that will get them votes.

That’s your reasons why.