Because another game is being played in parallel to the direct one you're thinking about. In the parallel game, if you let people who expect good salaries go, you do go through more immediate replacement costs, but are likely to eventually come across people who aren't so demanding of their salaries and get an accumulation of those, then the employer can win out in the longer term.

I don't know if this strategy even has to pay off in actuality, in the long term, but that actual performance of the strategy isn't as important as maintaining that model of control of labor costs.