Why is almost every OS people on this sub develop POSIX-based?

I've thought of starting my own project before, though I am impatient when it comes to reading books like the Intel manuals, but seeing people only making POSIX based OSes makes me wonder why. The OSes I have thought of making are a cross platform version of DOS including a JIT, and a cross platform version of classic MacOS, also including a JIT. Is making your own APIs difficult?