The services guard their missions jealously. They don't want competition within their various roles -- those roles are their justifications for budget, status and involvement.

There are good reasons for specialization, of course. But the institutional politics can't be ignored.

I've heard that the Marines actually have really good air support, because they have their own planes and the generals make damn sure those pilots put the bombs where the grunts need them. I doubt the Air Force likes that. But the Marines are part of the Navy, and the folks running the Navy aren't interested in ground combat. They've already got an air role, on the carriers, and a strategic bombing role, in the missile subs, so they aren't likely to encroach much on the Air Force. Since the Navy puts a cap on how far the Marines' mission would grow, the other services can live with the Marines mixing things up a bit.

It also helps that the Marines have a PR operation unlike anything anywhere on the planet.

And they might be a natural home for the A-10, right? But you can't fly an A-10 off a carrier.