Because it doesn't make sense to waste thousands of tons of fuel to recover what is basically metal from the ocean floor.
This kind of almost religious view of the environment is exactly the same error behind some incredibly stupid decisions like germany getting rid of their nuclear power plants (and now they are burning coal, genius!)
It is not just money. Wasting fuel has a far more damaging impact on the nature than the negligible impact of having some metal tanks in the bottom of the ocean.
It is not like sea water is distilled water.
