This is believed to be the quantum explanation for the observed stability of matter. There are very large, perhaps insurmounable, difficulties in combining what works for particles with what works for gravity into one whole working theory. The excitation modes of a quantized string that give rise to elementary particle states are like the vibration modes of a guitar string that give rise to different notes. Particle physics is good for describing Nature but general relativity is on shaky ground.
General relativity is good for describing Nature but particle physics is on shaky ground.
Each theory works well separately, but they don't work well together.