Why not make the underground habitats on Earth? If you're going to live underground in a fully self-contained ecosystem why bother traveling to a distant planet?
Or on the surface. The oceans are really big and there's a lot of the surface that has basically nothing going on.
Of course this highlights the problem with all of these habitat ideas: They are placed in locations with limited or no easily accessible natural resources, so their options for sustaining themselves are limited. Obviously you need some farmers and janitors and techs to keep everything running, but beyond that what are they going to do? Building stuff is kind of out of the question because you have to ship in all of the raw materials. So your industry needs to be virtual. Coding, finance, etc... But the world only needs so much of that, and if you keep building colonies you're going to saturate the virtual markets while driving insane demand in the physical markets. The economics are hard to reconcile.
