GR and QM contradict on such major points as energy, with one saying that energy is everywhere and infinite and the other saying that Gravity is energy. Combining the two theories would yield infinite Gravity everywhere...which we don't see (nor could we, life couldn't exist).
The two theories are mutually exclusive. They could both be wrong or just one might be wrong, but both can't be correct...which makes String Theory's attempts to unite the two theories absurd.
You misunderstand. String theory does not attempt to unite both theories so that they both keep their complete validity, but to do so in such a way that they appear as limiting cases of a more general theory, so that one or both are technically "wrong" but not in a way that contradicts current experimental evidence.
This is a good thing to try to do, but the string theorists, having failed to do it, refuse to admit it.