Good articles at Slate today.
When Did It Get So Hard to Fire Teachers? today's Explainer asks. They give the long-form answer. The short-form answer is the title of this post.
I'll continue with my cultural explanation:
Back in the "good ole days" that Libertairans and Republicans like to mythologize, the reality of our nation was blatantly feudalistic and racist. Teachers had to unionize and fight, and fight hard, for the right to be fired for cause.
Because before then, they could be fired because for any reason, from not doing their jobs to not doing the principal when he demanded it.
When laws forced school system administrators to actually state reasons for terminating employment, it kinda sucked to be a school system administrator. There is, after all, no fun in actually educating children when compared to handing out political favors and yelling "off with their heads."
Since then, every problem with schools has been blamed on the teachers, because somehow, requiring a legitimate reason to fire a teacher eliminated all system administrator ability to "improve schools."
So, instead of working with teachers to create better teacher training, support them in the classroom, and reward good teachers (who show results) with tenure, the politicians and administrators decided an adversarial relationship with teachers was more appropriate.
With a robust adversarial relationship developed, teachers' unions concerned themselves mainly with opposing school system administrators and vice versa. Each political side had a stake and a narrative. And we all know how institutionally effective our systems work when politics and narrative are at stake.
You'll notice that actually improving schools and education becomes a secondary or tertiary concern, if it is a concern at all.
This is why test scores are so important from a policymaking standpoint these days - they are the only "legitimate reason" most incompetent school system administrators can employ to fire teachers. Otherwise, they might have to prove their own competency by actually evaluating the employees and systems they are responsible for.