By law the Army isn't allowed to operate fixed wing combat aircraft; that dates back to the founding of the Air Force as a separate branch. As for the Marines, their combat aircraft and helicopters are all capable from operating from amphbious ships or aircraft carriers. Part of their mandate for amphibious operations. The A-10s can't do that.
There’s a loophole in the Key West Accords: the Army is allowed to operate fixed wing battlefield observation, surveillance and recce aircraft.
Which can be armed.
Example is the OV-1 Mohawk (no longer in service).
For many reasons I believe that ground support should belong to the forces actually on the ground. Hog pilots apparently love their job and the plane itself but just about everyone else in the AF - not so much.
And By law the Congress isn't allowed to fund a military branch other than Navy and Army.