Because it is. Linux doesn't really have a concrete idea of a "base system" like the BSDs do. Linux is more of a hodgepodge of components that are developed by different, and often a lot more isolated teams than we think, that all just gets integrated together. Which is truly an astonishing achievement of engineering, so I don't wanna seem like I am short selling it. Think of like the developer who work on gcc and the libc and the kernel, maybe some cross pollination, but not a lot. FreeBSD, the user land, kernel and even the libc team all happens under "1 roof."