Because what you think is true today may not be true tomorrow. You don't know how it affects the ocean or the life there and the ripple effect it may have on the ecosystem.

This is effectively the same mistake as the people back in the day made, thinking that the ocean is so big we cannot impact it.

The half life of many of these materials are very long time and it is extremely ignorant to think that highly radioactive materials won't affect ocean life during that time.

Also, the material may be used as fuel for modern reactors in the future. Now you have dumped a lot of shit that is poision to basically all life which could be used to power society.