There's a lot of incentive. The problem is HR and employers have decided to turn a blind eye to it in favor of looking at the next quarter.
Why give someone a 10% raise because the market has spoken when you might be able to retain them with a 5% raise? Even if you hire new employees at a premium, some contingent of older employees will stick around just because switching jobs is a hassle. This sort of thing drives down quarterly costs which ultimately makes you look good to shareholders who can't see the internal destruction.
I agree. I don't think that there's any incentive to do that, though.