Lobbying/inter-service politics is part of it. In WW II, the separation paid off. The British Fleet Air Arm flew lightly modified RAF aircraft, which were grea

Lobbying/inter-service politics is part of it. In WW II, the separation paid off. The British Fleet Air Arm flew lightly modified RAF aircraft, which were great for many things, but not for carrier operations. The US Navy would not pay for aircraft that could not take off from and land on carriers very well. In the 1960s, the Pentagon imagined the F-111 as an all-purpose, all-service plane. Some USN captain told a congressional committee that it would not work, so becoming a hero to his peers and losing an prospect of promotion to flag rank.