There is really no point in trying to use reason here.The Dodgers are spending over half a billion on their starting rotation already and they have 6 guys for 5 spots in their rotation as it is. Ohtani, Snell, Glasnow, Yamamato, Sasaki, and Sheehan. Plus River Ryan made his debut last year and is their top SP prospect.
I wouldn't put it past them to add another big ticket guy just because they can, but they clearly have other needs.