They want to keep him stretched out, if not on a starter's schedule. He was available today but it would have been another low-leverage spot. Of course, what are the odds he'll be needed tomorrow?
The Phillies have only played three games since his last appearance, and before that he was still making a start. A bit soon to think he's been buried.
It doesn't really help him or the team if he just becomes a regular reliever and then they need him to start or even go back to a six-man (which they will eventually consider at the season wears on). He's already pitched more innings this year then he did last year (and it was zero in 2022) and also isn't accustomed to warming up like a reliever, so it's fine to take it slow.
More to the point, the team is succeeding.