Why would people move to Detroit for lower salaries to live in Google provided housing? They would just find other jobs in the bay.
Hypothetically if you were planning this as Google you wouldn't make the 1-1 scaling of lower salaries, but if you as employee would pay 80% less rent, could buy a massive home after working for <1 year, and still have more left over than you would ever have working in the bay, the trade-off would at least look somewhat appealing.