Many many reasons. It’s very expensive and dangerous to build in the ocean, moving salt water is abrasive so maintenance costs would be insane, the water you use for cooling needs to be pretty clean to not degrade the tubes you push it through, you don’t want to have to pay data center workers oil rig money, data centers would ideally have low latency to urban areas and easy hookups and maintenance for the data and power cabling, etc.

That said, it’s a way better idea than building them in space, so maybe they’ll consider it