Because it doesn't hurt to that's why.
Getting moved up before you are ready can hurt several things:
1. The players confidence
2. The confidence of other players who were more ready and were passed over.
3. The trust relationship between players and management that says: if I perform, I will get a shot.
4. The bottom line. Sending players who aren't ready isn't fun baseball to watch, that often results in empty seats.
That is the quick list of the things it can hurt.