A quick and simple reason is inequity; the good ones work harder and the under-performers are left to continue to under-perform with no management direction or consequence.
So what should we be doing, at a time when managers ill need the additional workload of a people management issue?
Again, the simple answer is to look at what the business needs, direct people to achieve it, provide them with the support and management to achieve, and reward when successful.
It does sound simple, but the work involved in setting your team to achieve what your business is looking for takes time, a good understanding of the business needs, and the ability of your people.
Once in place, however, a good recruitment manager can reap the rewards from a successful team and drive the business forward. The effort then comes from maintaining the success; you’ve worked hard to get there, you’ve recruited the best candidates, so what do you do to retain your people?