Why do hospitals give sign-on bonuses to new hires but not retention bonuses to existing staff?

That’s a good question and one that is often asked by existing staff.

This has never made sense to me. Mathematically, giving a retention bonus to existing staff is more cost effective when you consider the the time and cost of training a new employee. I wonder if there is some type of write off envolved, but even then, a retention bonus would save the company money.