People often stay to build up their fuck you fund, and to prepare their exit. Finding the right job can be a long process. And even when you find it, taking it is not necessarily the right decision. If you're married and you have kids, for example, that Apple offer might not be a no-brainer if you have to move to a new city. Also, once they've been burned by a shitty corporation, people can become very picky on the job hunt.