Thing is, there are a lot of things in the way of anyone just using what is already there... such as anything that's copywritten that they would need to buy or obtain permission to use. So the answer to this is yes, it is quite a hassle.
It is however possible to create everything from beginning to end but this requires all the know-how of doing everything from modeling, rigging and texturing to making a GUI and Art for the game to coding the basics to making the creative tools to the complex coding of all the network stuff and being able to set up a server for it while also making sure you've got things in place to prevent problems (such as a chat filter perhaps...). None of this is easy to do and can often take years, especially if you want it to be bug free and pretty.
On top of that everyone would still be going about their lives and anything could happen that would slow down or halt the project entirely.
Hope this explains why it isn't easy and why it hasn't been done yet.