CPS pays an average of just under $75K to teachers, which is more than most private schools do. Along with the extra job security and (promised if not delivered) pensions, that makes the teaching positions attractive to quite a few people. The teachers I know also feel good about dedicating their professional lives to students in CPS, who are generally in need of every bit of help they can get.
If I had made a bundle in the dot com bubble or something, I could see myself teaching CS in CPS. Or at least trying -- I teach grad school and don't know if I have the personality for younger students.