One major problem is that managers tend to be judged by how many people work for them. You'll often get paid more if you have a 20-person department that accomp

One major problem is that managers tend to be judged by how many people work for them. You'll often get paid more if you have a 20-person department that accomplishes nothing than if you have an extremely effective 5-person team. Combine this with the fact that most people hate firing someone - it's just an awful experience - and most managers won't fire anyone unless they're actively destructive. The owners or CEO would want line managers to fire do-nothing employees, but they're not the ones making the decision.