Because aircraft turned out to be very very very good at killing ships in the 1940s, and the best way to deliver those (short ranged, when you compare to the Pa

Because aircraft turned out to be very very very good at killing ships in the 1940s, and the best way to deliver those (short ranged, when you compare to the Pacific) planes to where your enemies ships happened to be was to build an airstrip on a ship and then send that ship relatively close to the ships you wanted to kill. And because ships are owned by the Navy, and the planes flying off those ships are killing the Navy's direct opponents, and the actual aircraft in question are specific to the Navy... It makes sense for the people flying the planes to be Navy too.