I think the idea of a sandbox game is that you make up your own story. For example, starting the game with the "player's character is in the basket of a hot air balloon" would not work well for a medieval server. The developer's job is to just put the tools in place for you to build your own world.
Fully agre with you yahwho. Though i also like the idea to have some kind of background. Thats why maybe it could be done by combining multiple factors and then get some background story. With the option to indiviudalize it of course. Would be nice