I think it's because most companies know that change is difficult and painful to most people. There's an "activation energy" that it takes for you to switch jobs, particularly if you'll need to move to a new location.

I worked for a company that exploited this very successfully by having an office in a city where there were not many alternatives for comparable work. Anyone they recruited couldn't really change jobs without selling their house, uprooting their spouse's life/career, and sending their kids to a new school. The company had a strong incentive to pay you to move there, but it cost very little to encourage you to stay.