I remember having bad teachers growing up. They always knew when they were going to be reviewed, and worked to make that class better. Drop in visit? No problem. "Alright class, put those worksheets away. Now we're going to ..." I know there are good teachers. I had a couple. Most were eithet inexperienced or burnt out. Then there were the few that were plain bad.
However, I see my point of view is not popular with teachers. That in and of itself leads me to believe it is the inability to accept criticism at issue.
When teaching is compared to other professions, few have the independence that teachers are privileged to have. Doctors also have it. Salesmen. I'm sure there are more, but I can more readily think of professions that have closer scrutiny. Just saying.