The market for cellular modules is very competitive. From a time to market point of view is cheaper and faster to reuse smartphone-grade socs instead of reinventing the wheel.

As far as I know, only u-blox is producing their own SoC (albeit for cat-m/nb-iot only) all the others are using existing solutions from Qualcomm and others.

This is only scratching the surface. I did not talk about certification which is another beast altogether.