Miller has a few injuries in his history including a UCL sprain. He's started some games in the minors, but he's never gone very long. He's just not a starter.
Much of the reason he can consistently hit 100mph and have such a good slider is because he only throws 20 pitches a game. If he threw, 80, he'd probably lose a few mph and some of the ability to throw his slider. Daniel Palencia has a slider that grades out really good on Stuff+ and hits 100+mph and he isn't a SP either (anymore, his issue was really walk related, but he also wasn't averaging 100mph in the MiLB when he was a SP either).
Just is what it is. It's what makes someone like Skenes and Skubal that special that they can carry 100mph into the 7th. They're insane.