When a US aircraft carrier puts to sea, it's protected by the latest anti-aircraft weaponry and a screen of escort ships and its own air cover to intercept incoming threats miles away.
Is any of this true for a lowly tanker? Not hardly --- and yet, the carrier and its strike aircraft won't work without the fuel the tanker provides.
So which target would you aim your antiship missile at? The hard target, with all its screening vessels with the latest defenses, or the slow, highly flammable soft target which is alone and virtually unarmed?