[pjmlp]

They offer a POSIX like API on top, which isn't the same thing, as the key APIs, and overall system architecture, are something else.

Also mostly because as it happens in most hobby projects, people keep wanting to replicate GNU due to the existing software, thus keeping the UNIX cycle alive.

[ori_b]

The POSIX API comes with a large number of warts and constraints, and requires a great deal of specific machinery to support.