I don't know if this is a definitive reason or not, but when applying as an Army Officer that is exactly what you apply as. It is only once graduating from Sandhurst that you are put into a regiment, which can be anything from Air Corps, Paras, Cavalry etc. There are tests you can do before you go to Sandhurst, and obviously have a preference for one particular role, but just because you want to be AAC doesn't mean you won't get stuck as a tank commander if that is where the requirement is. It's very much Army Officer first, then whatever trade you are afterwards. This also explains why NCO's can apply for aircrew, as the more pertinent role in the eyes of the Army is the soldiering skills.
This is different to the Navy & RAF, where you apply as direct entry aircrew so will do your FAT's at Cranwell before OASC/AIB.