I just get annoyed that they are talking about being "understaffed," but we can't get all of the regular characters involved on screen at once. We have to find creative ways to write Rollins and Carisi out.
I'm guessing it's a budget thing. Kelli Giddish has been on the show since season 13, so she's almost certainly costing a pretty penny by now. Peter Scanavino hasn't been on quite as long, but I'm sure he's got escalators active by now, too. Easier and cheaper to have one-time guest stars.