Because of the GPL, GNU/Linux does not fragment, as Unix did.
Of course it does. You think say Android's version of Linux is the same as the Linux mainline? Or Oracle Linux's version of the Linux kernel is the same as the mainline?
Android's version of the kernel is available in source form, and there are rumblings of an effort to integrate it into mainline. Even if this doesn't happen now, it can happen in the future because the source is available, unlike computer manufacturer's proprietary versions of Unix.
I don't know anything about Oracle's kernel, but unless they are violating the GPL their source is also available.