Only because it is usually very hard to determine how the current projects will be affected if one gets rid of a particular manager. So the next level manager will always take the path of least resistance / least risk and move the person to a different role while having some smooth transition on projects. In reality, more often that not, this is completely unnecessary. But the person making the decision outweighs their project’s success more than the wastage of an unproductive employee. Basically a form of kicking that can down the road.