Why warships are not built from stainless steel

Why are warships not built from stainless steel?

Because stainless steel is extremely expensive and difficult to weld and form at ship scale, despite its corrosion resistance.

Stainless steel offers strength and resistance to salt-water corrosion, but whole-ship construction runs into the material's very high cost and the difficulty of welding it into the required forms. A warship material must be judged across procurement, fabrication, repair, and structural performance rather than by corrosion resistance alone. The industrial constraints turn an apparently simple material choice into a much harder design problem.