I just do this for fun: there is a massive gap in effort, cost, and stress between working on a modification of an already complete game, where I don't need to bother much about dealing with people, and actually starting from scratch to program everything, needing to appeal to the most common types of player to make a profit, dealing with advertisement, deadlines, complaints, and so on.
"Single player PW" might seem conceptually simple, but it would be much more trouble to implement than multiplayer PW, since a large part of the interesting "content" is made by the human players involved, needing only a light framework of game rules and features, rather than a complete and detailed implementation of AI, with enough variation that interactions don't quickly become repetitive and boring. There are many other games and engines more suitable for that type of single player experience, such as "The Elder Scrolls" series, Morrowind, Skyrim, etc - which all require massive development teams to create an immersive world.