Because he exactly fits the profile of starter turns into RP. Walks don't hurt as much as a RP, you can throw even harder and fastball+slider is a viable combo. Peralta screams bullpen guy with every single skill he shows. I don't know about closer since he can't get out lefties, but he would be so much better in the pen than as a starter. Guys with 2 pitches just don't do well as starters. OPS by time through lineup .725, .754, .818. OPS at 75+ pitches is .871. He doesn't have the stamina or arsenal to be a starter.