Simply because the Traditions of the Air Force (in the United States at leas, though also in every nation that I'm aware of) come from the Army since it was a part of the Army until 1946. Prior to that it was the United States Army Air Corps. The rank structure was taken from the army and adapted to fit the needs of an air service. All of the officers had been Army officers until then and all of the enlisted men had been in the Army. Naval Aviation had very little direct influence on the creation of the Air Force.

The usaf was formed out of the army air corps and thus borrowed much of the army's traditions, including commissioned ranks.

That aircraft and seagoing vessels share some common terminology is more related to historical French advances in aeronautics long before the Air Force was established.

Sailors are happy to point out, however, that the worlds 2nd largest Air Force is in fact the U.S. navy- so it could be argued that admirals do oversee certain air forces.

They didn't. The RAF, the first independent air force, adopted it's own system of ranks from Pilot Officer up to Marshall of the Royal Air Force. None of these were army ranks, indeed, Air Commodore is clearly based on a Naval rank. http://www.raf.mod.uk/organisation/commissionedranks.cfm