Comment by delfinom

It takes an astronomical amount of work to not only write an OS but handle 30 years worth of both hardware, and lessons learned.

People do write new OSes from scratch all the time, you can see the result of no popularity/usage because they can't do much for even enthusiasts.

Not to mention, if you want users, you need apps, and that involves trying to run what already exists or making it as easy as possible to port. All of that shapes the OS.