Because Windows and Linux are not designed for hard real time. There's a "rt_mutex" thing in Linux, which does priority inheritance, but it's not used much.

QNX took this seriously; it has priority inheritance not just for mutexes, but for interprocess messaging and I/O too. But it's a real-time OS. It passes through not only the priority, but the CPU quantum for non hard real time tasks. So a group of rapidly interacting processes are effectively scheduled as if they were one process which happens to cross address spaces.

On most non-real-time OSs, if you have a microservices-type program which is rapidly passing control back and forth between multiple processes, it will work fine as long as there's spare CPU time. If there's no spare CPU time, performance suddenly drops badly. That's because the processes being released by mutex unlocks go to the end of the line for CPU time.

This is something microkernel designers take seriously, because it has a big impact on microkernel performance. Unikernel designers, not so much.