I think there is a lot of room for competition in the accelerator space e.g. GPUs, ML inference engines, video encode/decode.

It seems to me like modern core design is expensive and high risk/low reward for most of these companies. Remember, Apple has a ton of R&D and the margins to put massive cores into just about everything they sell, upto and including putting a CPU faster than most desktops into their "budget" monitor. Fabbing costs keep going up and the extra revenue from larger+faster CPUs would get gobbled up by TSMC and Samsung Fab. Setting out to build something faster at the same size and R&D is what eats your profit.

For Qualcomm, Samsung, Google (for Pixel at least), and NVidia, ARMs balance of performance/die size/efficiency is aligned well with their market positions and the opportunity cost of rolling their own is just too high even in the face of thinning margins from competition.