In Malaysia, it comes from the many government teachers who do their job poorly, and really don't merit the title of teacher. In their defense, the pay is usually around 2k rm / month, which doesn't really buy much.
In the States, it comes from a lot of the right wing talking points. The horror stories of tenured teachers.
When I was going back through the pics I've taken so far this year in Malaysia, I found this one.
This was on a Sunday morning. A stack of exams. Lessons being planned on the computer. And my butter coffee.
I also worked on Saturday. I also work most nights, late into the night. Teaching is, realistically, for the serious teachers, a 12-14 hour a day profession.