I don't disagree with you OP, in that teachers are generally amazing! Do a great job and are very very needed.
However, YABU because this is a forum largely used by parents. One of the biggest parts of their children's life is school. Parents worry about school and want their children to be happy and safe there. Mumsnet is a place parents can come to air concerns about teachers, let off steam, moan etc.. without doing this in real life because sometimes they know that wouldn't be fair.
This is a forum, views will be expressed. A forum largely aimed at parents will naturally discuss schools and teachers.
If you are not happy on here OP, you can, as you have done, say so. You can also find other forums to visit instead.
If you are a teacher, thanks for being part of a great profession.
The number of teachers talked about negatively on here is a mere pin prick. The rest (a vast sea of them, millions I'd estimate) are not mentioned on here at all because mums and dads across the country have nothing negative to say and are happy. That speaks volumes