I don't think the situation is that different. Linux being the kernel, there are lots of "Linux distros", some of which are almost as different from each other

I don't think the situation is that different. Linux being the kernel, there are lots of "Linux distros", some of which are almost as different from each other as FreeBSD and NetBSD. For example, consider the many package managers (deb, rpm, Nix, etc.), init systems (SysV, Upstart, Systemd, etc.), graphical environments, etc. The BSDs have a different philosophy where the kernel and userland go together as one package. Linux distros reuse the same kernel and rely on GNU for many tools, but are otherwise free to do their own thing, just like the BSDs. I doubt the GPL is what really keeps Linux together. Rather, since the Linux kernel is developed separately from everything else, and is such a huge thing, there's little incentive to fork the kernel along with userland. You develop a new userland (as with Android) and keep the kernel.