Aging veteran starters and rentals are the only ones who usually get moved mid season and often cost a lot. They can be useful as we’ve seen with Eovaldi and Peavy but it’s rare to see an ace move or even a young pitcher with control like Woo.
Even when Castillo was moved, I think he only had a year and half of control left and at the time people thought the Reds got a good prospect haul. However, I think most of them haven’t worked out and Marte has battled steroid suspensions.