Because the thing I described is still more flexible than having no OS at all. If you bundle some of the userspace into the kernel image, then you trade-off so

Because the thing I described is still more flexible than having no OS at all. If you bundle some of the userspace into the kernel image, then you trade-off some security, and needing to have to rebuild this large image periodically, for much faster startup times, and (optionally) load times for some userspace applications - but you can still install and load other userspace tools, they'll just start up "normally".