He isn't being paid like a number 1. Kershaw and Scherzer are number 1's and they're making 30 million per year. Furthermore, Porcello's ERA looks far less damning when you look at his FIP and consider the fact that the Tigers set him up to fail by giving a guy who pitches to contact an awful defense behind him.
Edit: also, I'd rather have the team paying big money for what might be than what has been.