[ghaff]

The thing is: You're now effectively living on an island, albeit one that requires ongoing maintenance and other expenditures to remain seaworthy. And hope you don't have a big storm that you're very exposed to. Most people don't want to live someplace where they have to take a boat trip to go to work or the grocery store.

Some are fine with that but, guess what, you can often live on islands with a fairly low COL. There's tons of land in the US generally. It's just some specific locations of land that are expensive.

From an engineering perspective, you also can't build it up that much. When I worked on rig designs, every time we added equipment, we had to verify that the center of gravity remained within acceptable limits. You can't just build a multi-level structure atop the entire rig or it would become dangerously unstable.