Because the kernel is the irreplaceable piece. None of what GNU did is: there are numerous implementations of coreutils and shells and at least one non-GNU production-quality compiler toolchain (clang-llvm), a few alternative libcs. And many distribution do actively use the non-GNU parts. But none of this is useful without the kernel that is compatible with computers people have. And the only usable kernel we have is Linux (while BSDs are out there too, they take a much different tightly-integrated approach to userspace).
To add to this: I can appreciate the significance of GNU, especially in early Linux distributions, but the position of "GNU was the real OS, Linux was just the kernel" is also deceptive, IMO.
Sure, a lot of the userspace was GNU, but a lot of it ... wasn't. Things like PAM, the init system, and the network config tools, off the top of my head. A lot of system-specific tools come from "not-GNU", too.
You can't discount how much of early Linux was "GNU", and how big a deal GCC and GNU libc (and the rest!) were, but it's disingenuous in my opinion to call GNU an "operating system" that you just plugged Linux, the kernel, into. Even today, as far as I can tell, there is still not a true GNU system. Guix comes close, in terms of being "GNU-ish", but the most usable Hurd distro (AFAIK!) is Debian, where, again, a lot of components come from Debian, rather than GNU.
And, as you say, modern systems have drifted even further from being GNU. They have lots of GNU components, but so did, say, the Sprite OS, or a lot of 4.4BSD derivatives.
On that note, one of these days I want to make the GNU system as it was imagined a reality. Perhaps with the linux kernel as its kernel, maybe with co-official status with the hurd
Oh, I don't discount that! That's why I find it important to specify GNU/Linux. Not only is it respectful, but it makes the very important distinction that it is a Linux system running a GNU userland instead of a plan9 one or a busybox one. Usually when people speak of "linux" they're referring to GNU/Linux though
