Why would it cost so much to scrap a ship?
Enterprise was overbuilt for her design. She was the first thus kind of a “demo”. She had EIGHT nuclear reactors... EIGHT. They essentially have to slice into the ship in order to defuel and remove them. This all needs be done before a scrapping yard will even touch her.
Nuclear-powered vessel. Radiation. Eight reactors that need to be removed BEFORE breaking it up for scrap.
Doing a SinkEx on it would raise nine kinds of Hell from enviro-weenies about sinking a 'radioactive' ship in the ocean.