I think complaints can be useful in identifying a problem, but if your complaints have been made, just continuing to complain doesn't do anything. On the other hand, quitting, does two useful things. First, it takes you out of a bad situation letting you move on to a better one faster. Second, it helps brings the problem to a head. The administration can ignore teachers complaining, they cannot ignore teachers quitting. At some point they will have to resolve the issue.
They are.
The exception tends to be at the poorest schools where the principals have trouble hiring and keeping teachers. When I was interviewing for teaching positions, I met with the principal for a school in South Central L.A. and the only question he asked me as, “do you promise not to quit before the end of the school year?”
Elsewhere bad teachers are fired, sometimes even in the middle of the school year.
Plus, my own experience in the public school system as a student tells me not all teachers are working 12, or even 8, hour days. Again, if you want to be a good teacher - why complain that it's hard? Just be the best teacher you can with eight hours of work. It's not like teachers are fired for being bad.