Probably for the same reason cops might be hated. They are the ones you get sent to when you break the law at school. Not an easy job, being the disciplinarian. Teachers are hired to teach, and have to send kids to the principal for discipline. Principals have to track down truant kids and deal with parents. They have to deal with all the complaints. They have a difficult job.
Not all principals are hated though. When I was a teen, I had one who was female, friendly, interesting and very personable. I recall talking with her often, as she hung out during lunchtime in the cafeteria. This same school had a line of boys, most days, waiting to visit with the principal (and vice principal). I'm sure those kids feared her, but I thought she was nice.
I have worked at various schools, and many times I have seen very friendly principals, visiting with kids and getting hugs too! Then again, I've encountered those who are stand-off-ish and always frowning and seem unapprochable.


