It’s a societal failure. U.S. teachers generally come from the bottom 20% of college students. These bottom 20% teach the next generation of students, who end up worse than the previous generation. Now we take the bottom 20% of this group, and so on and so on. We’ve been through about three generations of this process since public education was any good, and what you see is the result.
Finland is praised for the success of their education system. There, teachers are selected from the TOP 10% of college students, and their results show it.
(All info presented is statistical. Of course there will be outliers in either situation.)
The unions try to claim that this is because teachers don’t make enough money, but these days this isn’t true. They get 3 months off a year, guaranteed pensions, tenure after working a few years so that it’s impossible to fire them even if the molest children, etc.