POSIX doesn't speak to the problem at all (except for chroot). It's just not part of the standard.

With linux, it's because the tools are exposed piecewise. Creating a process namespace isn't the same thing as creating a virtual network device. You can use cgroups to stop a container all at once, sure, but you can also use them for other useful purposes and the code is the same. Some of these APIs are simple, some less so. But they're tools.

I know, I know, now you're asking "well, why didn't someone just put all the tools together in one box that would do it with one sane UI?!"

They did. It's called Docker.