Why cannot we add gravity in quantum mechanics without additional dimensions as done in bosonic or superstring theory?

Question

Is it because Einstein described gravity not as a force but as a curvature of spacetime? Physicists usually say that it is hard to combine quantum mechanics with gravity. But I had doubt, is it because of Heisenberg’s uncertainty principle that quantum particles can be at multiple places at once and therefore the gravity associated with the electron (quantum particles) can be at multiple places? Or as Hawking suggested in his book “A Brief History of Time” that when we work to do the math we end up coming up with numerous infinities which does not gets cancelled, but the more successful theory of quantum mechanics also has infinities under singularities but still we call it a successful one, so can’t those infinities get ignored (And what those infinities even mean as no one clearly tends out to show us the math behind it). I strongly suspect that there is an incomplete understanding of quantum mechanics in me as for sure the great physicists cannot be wrong if the is, then can you please explain. Thank you. We all have heard about the genius of Einstein about description of gravity not as a force but as a curvature of spacetime. We have encountered numerous situations in which Einstein’s general and special theory of relativity have provided us with results with extreme precision then why are we searching for a particle associated with gravity when we know that gravity cannot be expressed as a particle as it is the mere bending of spacetime? Your views ..