I can think of a few reasons.
1) They are short timing it till retirement.
2) They don't want to join a labor union.
3) They like their bid.
4) They like their seniority and don't wish to give it up by starting over.
5) They have found a way to get along with mngmt and don't get picked on.
There are other reasons, I'm sure. I hope this helps, a little bit, to un-boggle your mind.
