Possibly licensing reasons. Linux is GPLv2 only, Hurd seems to be GPLv2 or later, there could be reasons you may want to use something under the GPLv3.
That's debatable, since what people generally call "Linux" is more GNU than Linux anyway. "Linux" as the Linux fandom considers is it big and professional like GNU, because it is GNU (among other things).
But what about Linux distributions compiled without GNU tools? Most popular Linux distribution's kernel currently is compiled with Clang, not GCC, and as far as I am aware does not include anything from GNU. Of course Linux is historically influenced by GNU, but in current day and age they are orthogonal
It doesn't change the larger point that GNU is way bigger than Linux, though.
There are a tonne of things that are larger than Linux, and GNU is one of them.
I guess this is a matter of perspective. What I was saying in my previous comment is that what people commonly refer to as "Linux" (as in "Linux distributions") is not just Linux (which is just a kernel) but also includes a bunch of other stuff, including GNU (that is what GNU/Linux refers to). If you're talking about the actual thing called Linux, you'd be right, because most GNU systems are GNU/Linux systems, whereas arguably most Linux systems are not GNU systems; Alpine and Android are non-GNU Linux systems.
However, if like many in the Linux fandom you discount Android, then most Linux systems are GNU systems and vice-versa.
I wouldn't either, but you see the sentiment especially among the [email protected] crowd. The fact that Android is Linux is significant because it does in fact allow you to run so-called "Linux apps" (either under Termux or under a dedicated chroot set up by something like UserLAnd) and that is something to be lauded. Android has problems but not being "real Linux" is not among them.
I mean the GPL allowed linux to become a commercial entity. And the whole "professional" outlook is because theres a ton of companies who contribute either funds or development to the project.