The USAF doesn't have attack helos because they are designed for taking out tanks and other ground targets and supporting troops on the ground. Of course the obvious question then arises, why is the A-10 Warthog a USAF bird and not Army? I don't know the answer to that.
Back during Vietnam, the Air Force got in a huff about the Army using armed observation aircraft in the air support role there instead of depending upon its jets, so ever since 1965 the Army is not allowed to fly armed fixed-wing aircraft.