Naval treaties and acceptable cheating

Why did major navies stay in naval treaties if no one could truly police them?

Treaties helped governments control costs and reduce war risk, while also creating a mutually understood limit on how far states could bend the rules.

Great powers remained inside naval treaty systems partly because governments wanted to redirect spending after the First World War and feared that unrestricted competition could make another war more likely. British and American paper exercises against each other do not by themselves prove that either expected an imminent Anglo-American war; competent staffs train against the strongest plausible opponent. The treaties also did not make every signatory perfectly honest. They defined a mutually understood boundary for tolerable evasions and made behavior beyond that boundary strategically alarming. Arms limitation could still matter when enforcement was incomplete and every participant expected some cheating.