The difficulty is turning the metric tensor into a quantum field like object. Apart from being incredibly complicated, because the field interacts with itself and all other fields, there are a bunch of mathematical quirks that prevent this from working out like it does in all our usual quantum field theories. Until someone comes along with a theory where all this works and still returns our normal physics in the low energy limit, noone can say what might really happen at that level. Maybe spacetime isn't quantizable because it fundamentally isn't quantum. Maybe it doesn't even exist at those length scales and what we perceive as space and time are emergent properties of some yet to be understood quantum process.