Struxta is an extraordinarily advanced civilization, and the technology available to the androids is fairly out of this world! They have not dug downward out of a lack of necessity, and their buildings manage to stay erect regardless. Additionally, as the abandoned network below would create more problems than it is worth, there is not a real need for them to dig downward unless they require additional storage space for parts -- and even then, there's technology available to remedy that storage problem in other ways.

We designed Struxta this way purposefully. :)