Hospital ships and disaster readiness

Why cannot a navy simply deploy its hospital ships whenever a natural disaster occurs?

A hospital ship on sustained medical work is not immediately free, so a rapid disaster response requires additional capacity kept available for that purpose.

Hospital ships are often committed to long-term medical deployments rather than waiting idle. Keeping one ready for a sudden disaster means funding another ship that may appear underused in normal times. This is a readiness problem rather than proof that the ship lacks value. Armed-response, riot, and mounted police units face a similar problem because their specialized capacity is essential in a crisis but becomes a recurring budget fight between emergencies. Military forces encounter the same tension when they must preserve assets for emergencies while justifying them during peacetime.