In most situations, we can neglect quantum mechanics or general relativity (or both). The moon is too large to be sensitive to minute quantum granularity; so we can forget the quanta when describing its movements. On the other hand, an atom is too light to curve space to a significant degree, and when we describe it we can forget the curvature of space. But there are situations where both curvature of space and quantum granularity matter, and for these we do not yet have an established physical theory that works.
