Why navies rarely control their own budgets

When could a major navy dictate its budget from the capabilities and availability it was expected to provide?

Almost never, although the Royal Navy around 1909 came unusually close to exercising that degree of political leverage.

Navies have rarely been free to name both the force they want and the money required to sustain it. The early-twentieth-century Royal Navy came unusually close, illustrated by the political formula that the Navy wanted six ships, the Treasury wanted four, and Parliament compromised on eight. British appropriations historically allowed substantial discretion inside an approved budget, while Congress authorized American force structure and appropriated money for specific ships and programs. The comparison shows how constitutional and accounting systems shape fleet design as powerfully as naval doctrine.