This would be a fine argument if we could somehow align incentives, fire low performing teachers, and rate them retroactively based upon the outcomes of their respective students.
Why can't we fire them? The typical answer is "unions" but a bunch of states de facto don't have teachers' unions (as far as unions with any ability to collectively bargain or meaningfully protect teachers from firing), and still have to retain all but the very worst teachers... because the supply of good teachers is too low. These same states tend to have poor educational outcomes, further suggesting that ability to fire, on its own, isn't helping much.
> rate them retroactively based upon the outcomes of their respective students.
This, unfortunately, turns out to be a really hard problem. It's been sincerely attempted in a bunch of ways and they've all turned out to suck, is the short history of the notion.