I was wondering the other day why we need to find a quantum explanation for gravity? Why not just try to take the QFT and put it on a curved spacetime, and see what happens. That was how I came upon quantum field theory on a curved spacetime and semiclassical gravity. My question is (because I'm only a first year physics student, I dont really understand a lot of this), does this way of doing things actually work? Why can we not just consider gravity as described by Einstein to be fundamental, as we consider the axioms of QFT to be fundamental?