It's probably simply a question of the "lawless" open seas permitting absurdly excessive tragedy of the commons-style tradeoffs. You don't need to go all the w

It's probably simply a question of the "lawless" open seas permitting absurdly excessive tragedy of the commons-style tradeoffs. You don't need to go all the way to nuke-carriers; plain old combustion can be much, much cleaner than some ships at negligible (but non-zero) cost - witness similar engines/powerplants on land. But in an extremely competitive market, what sane operator would do that?