It doesn't make sense to pull people off regular operational jobs for projects. If their work was important then they need to be replaced. If it wasn't important then they shouldn't have been doing it in the first place.
Employees lack required skill sets.
Sometimes work comes up that regular employees are simply not quipped to handle, e.g., process redesign, unique procurement or long-range planning. Assigning people work they don't know how to do rarely produces satisfactory results. Training for one-time new tasks is not cost effective. Learning-curve expense is often overlooked. Employees are overloaded or burned out.