I think it's just more of a numbers deal. Minors are almost always off on Monday, so you have six starts the rest of the week, and South Bend has 15 active pitchers on their roster, and some of that will get worse as guys get moved around the system and guys are promoted. A bit of a new problem after baseball cut a whole layer of teams. As mentioned by
@hawkifann you probably have load management issues at work, too. You have to get all those guys work. Devers, who you mentioned, just came back from the IL. He was a break out pitcher last year, but needs to be carefully folded back in. The Cubs also a lot of piggyback starts, so one start a week isn't shocking. It's really about getting in set work.
A lot of people think Horton should be pushed to AA this year, but my amateur opinion is they will artificially limit him to about 100 innings, then send him to Arizona to work on mechanics and physical conditioning.