Why wartime landing craft needed many services

Why did several services operate landing craft instead of assigning all of them to one?

The fleet was so large that multiple services were needed to supply enough trained small-boat crews.

Combined operations used 4,126 ships and craft in the western theater, roughly three quarters supplied through American Lend-Lease. Comparable demand existed across the Pacific and other operational areas. Manning thousands of small vessels required a huge pool of people who already possessed boat-handling skills. Drawing on naval, coast-guard, and other service organizations was more practical than transferring every craft and crew into a single branch.