Why supertankers support a fleet but do not become one

Could supertankers become fleet supply ships or improvised supercarriers in a future war?

They can move supplies, but they are too slow and vulnerable for fleet work, while full carrier conversion is less economical than building a carrier.

Wartime merchant transport differs from ships able to accompany a fleet. Supertankers are too slow for fleet replenishment and would require escorts or containerized defensive systems to survive. Medium container ships may be useful conversion candidates, but turning the largest hulls into aircraft carriers demands so much work that a purpose-built carrier is cheaper. The exchange separates valuable sealift from operational fleet support.