Mostly the argument expressed by others here: it would be too boring and it will encourage the further production of things like No Man's Sky that will ultimate

Mostly the argument expressed by others here: it would be too boring and it will encourage the further production of things like No Man's Sky that will ultimately fail to satisfy because the number of hedons per game frame is very low, and the huge/infinite world aspect only makes it harder to increase the density. Outerra is pretty cool for what it is, but I don't see it as the basis for a new Second Life or even an MMORPG. Perhaps there's potential for something on the scale of Dwarf Fortress like populating the planet with billions of unique agents with history, or as an interface to the real world via data collected from the IoT tracking the geometry and movement of every interesting thing on the planet.