Because with Berrios, Santana, Odorizzi, and Mejia definitely ahead of him, and with Gibson, Hughes, Sanchez, and Slegers having more experience (and more dollars, for better or for worse), it's not as simple as you might put it, team-wise. Also, he's never pitched above AA, and we all know that we can't trust spring training stats to overcome a real track record. So I'm going to assume that that's not what you're doing. But speaking track record, he's never even pitched a full season. Also, even if he is the best option right away, the question is "by how much" because by leaving him down in the minors for a month they get an extra year of service time. Remember Kris Brant? I'm going to assume that you do. Is an extra win worth a year of a guy who can be an ace? I doubt it, but you may disagree. Also, if he gets injured he gets service time, and again, that's a huge waste. It's not about him being the best rookie. It's about whether he's among the best five right now, it's about how he fits into the organization, it's about how the decision impacts the team money-wise, it's about how the risk stacks up, it's about how much you can trust his stuff absent a real track record, etc., etc., etc. You act like it's a super easy decision not to option him. For a million reasons it's not. And that's why people are bringing it up.