In my limited understanding I had read or heard that Einstein developed GR from the Principle of Equivalence and conservation of energy perspective while the mathematician David Hilbert almost derived the same field equations from the Principle of Least Action. How do these fundamental physical principles arise within QM which Planck began around 1900 simply by making an off the cuff assumption that energy frequencies of radiation must be quantized in order to fit experimental data to explain the ultraviolet catastrophe? Could this be the underlying cause of incompatibility between GR and QM? GR is grounded in well established proven conservation laws while QM started off as an ad hoc guess simply to fit empirical data? Also while GR predicts a dynamically active curved spacetime which can be interpreted as gravity doesn't the current accepted QM framework act on a flat non dynamic spacetime?