There’s only so much funding to go around. Unfortunately, it’s just the way it works right now, so you have to be careful who you let work on your projects. If you don’t produce results, the funding agency is unlikely to give you funding next time you ask for a grant. Ergo, you have to be stingy about who you have work on projects funded by grants.

Source: My wife is a professor, and she always complains about the funding models she has to deal with.