Hmm, then what happens to outdated cables? Do we pull them up, or do we just turn off a switch at the shore and leave the rest in the ocean? Do the cables contain any substance that might pollute the seabed as the casing degrades away?
Plus, the impact from burning the fuel it would take to have a ship pull up a cable would be thousands of times worse than anything the cable might do.
I can't imagine what materials they would be made of that would be particularly harmful. My sense is they're mostly constructed of things like rubber, plastics, other polymers, steel, copper, and of course a little glass fiber.
As time passes, most of the cable is buried deeper and deeper under debris that falls on the ocean floor. This means that you cannot just pull it up, you would have to dig to get it out.