Meat of the story:

The root cause turns out explicitly to be because MMUs are so valuable: without one, you can’t run Linux, BSD, or Mach. Thus, when ARM split their IP portfolio into the A, R, and M-series cores, the low-cost M-series cores were forbidden from having an MMU to prevent price erosion of their high-end A-series cores. Instead, a proprietary hack known as the “MPU” was introduced that gives some memory security, but without an easy path to benefits such as swap memory.

The RISC-V Baochip has one.