Because pitching at max effort for 3-5 hitters a few times a week will not help him develop into a starting pitcher. He has to work as a starter to learn how to consistently command his fastball over extended innings and learn to command his changeup which he will barely ever throw as a reliever. He has a great arm but he's just a thrower right now. He still has to learn how to pitch and he will not be bale to do that working out of a major league bullpen. They're being greedy by sacrificing his long term value. I guess they're happy with him being a thrower when he could be so much more. It requires some patience which they have shown time and time again they do not have.