The other hard part besides hardware is that an OS is useless without useful apps (or at least a browser) so the OS dev also has to recreate the entire suite of

The other hard part besides hardware is that an OS is useless without useful apps (or at least a browser) so the OS dev also has to recreate the entire suite of browser/email client/word processing, etc etc which now requires even more effort, or some kind of window/linux compatibility layer, but in that case you might as well just spin off another Linux distro