Personally I have a different approach to these things. This is a bit of a different philosophy. Many string theorists do like the idea that they are in a sense discovering what's going on with the theory rather than putting stuff into the theory. I don't know if I agree with this, but it is a philosophy.
Some people have.
Witten, for example, has a '92 paper called something like Background Independent Open String Field Theory. This paper has turned out in the last month or two to give very important results in regard to the condensation of the open string tachyon. I don't think it's as if no one is working on it. The same comment applies. Well, there's matrix theory where it's difficult to even find a spacetime to be background independent of.