Why don’t they just go work elsewhere, where they don’t have to put up Michael Scott? Most of them could easily find other jobs. Why do they stay?
Habit is a big part of it. Every day that you do the same thing it becomes harder to do something different the next day.
Beyond that, everybody has individual reasons.
Pam stays in the Scranton reception desk for the same reason she doesn’t dump Roy. She has low self-confidence. She doesn’t think she can do any better.
Jim is in love with Pam, and stays where she is. He also likes thinking he’s superior to everybody else he’s working with—Michael and Dwight first and foremost—while starting to fear he’s no different than they are. And for Jim, Dunder-Mifflin is easy money.
Easy money is the lure for Stanley, too. He just doesn’t give a shit about office politics.
Dwight and Angela get off on their perceived power, and Dwight of course has a massive bro-crush on Michael Scott.
Kelly is oblivious, and in love with Ryan.
Ryan sees the office as a necessary stepping stone to a bigger future.
Meredith is a drunk.
Toby, like Pam, doesn’t think he can do any better. In Toby’s case, he may be right.
Left as exercises for the reader: Kevin, Phyllis, Creed, Oscar, Darryl, and the later-seasons characters.