People are only going to leave if they can get an equivalent job at a competitor. If and when the current tech bubble bursts, these companies will all have a hiring freeze at the same time, just as they're all in a hiring frenzy at the same time right now. We live in a feast-or-famine economy, and we tend to forget past famines while we're feasting.
If a company controls your paycheck, it's hard enough to leave. If they control both your paycheck and your housing, it's much harder to leave. If you want to leave and you put in your two weeks' notice, are they going to respond with notice that you're being evicted? So now you have to handle a job change and a sudden move at the same time?
Sudden? If you live in a company-owned house, would you really expect to stay after quitting?
If you want to leave, you plan your move along with your change of job. If for some particular reason you may think this will be exceedingly problematic, you can move before quitting.
The fact that people would leave? Same as what stops them from lowering salaries, which they can already do and would end up the same result.