Clearly the Phillies are prioritizing development, health, and a specific innings plan for Painter, McGarry, and Abel. Miller has been mostly hurt until this year but is now pitching well (1.52 ERA) in a multi-inning relief role.
Maybe he can build endurance and stamina in the future, but I really like him in this current role. Let's him develop more of his pitches, stay healthy, and actually get to the majors fairly quickly. He has good stuff. Left-handed, mid 90s velocity, decent slider and change. I think they are just being careful with him because he has pitched so few innings, but also remember multi-inning relievers are more part of the game these days. 2-3 innings every 3-4 games can be pretty valuable.
Edit - On Miller he just missed some time with COVID but he should be back this week.