I was also hoping that the article would explain why. I was guessing that with certain chemicals, it might dilute it enough to become harmless.

But instead, the closest I found was this, which cited cost as the reason: https://en.wikipedia.org/wiki/Submerged_munitions

I’d like to see a comparison of the long term economic costs of these accidents with just dealing with them early on. Or even entombing them the way we do nuclear waste.

I also found this article and map

https://nonproliferation.org/chemical-weapon-munitions-dumped-at-sea/