Because the POSIX standards are laid out and ready to be picked up and implemented. If I were going to learn about operating systems by building an operating sy

Because the POSIX standards are laid out and ready to be picked up and implemented. If I were going to learn about operating systems by building an operating system, the first place I would look for a checklist of what is needed would be the POSIX standards. After implementing something similar, I'd start iterating my project into something unique. If you don't know what you're doing, a rulebook is a great place to start.