Secretly, I suspect the answer is market differentiation. You can charge a higher royalty for a CPU core that has an MMU, and bundling it into the low end stuff

Secretly, I suspect the answer is market differentiation. You can charge a higher royalty for a CPU core that has an MMU, and bundling it into the low end stuff erodes margins. The complexity is real, but in a typical SoC the actual CPU core is maybe 10% of the area and tossing in an MMU impacts maybe 1-2% of the total chip area. I haven't seen the pricing sheets, but I suspect the much bigger cost is the higher royalty payment associated with instantiating the MMU.