Linux is only a kernel. It isn't a full operating system; it's only a small (albeit central) part of one. Everything else is components like the GNU utils, X and/or Wayland, so goes on. That's why some people call Debian, Fedora, Arch etc. "GNU/Linux distributions", to highlight that those components are damn important.
Android uses the Linux kernel (or a modification of), but it lacks practically every single other thing you'd see in a common GNU/Linux distribution. And applications are incompatible - you can't run Android applications in, say, Debian, nor vice versa.
And, more importantly, Android lacks what makes GNU/Linux worth using: commitment to free and open source software (FOSS).
Sure, you can fork Android. Some already did it; it's called GrapheneOS. But when people say they want a "Linux phone", they mean they want a phone version of a GNU/Linux distribution, because of that commitment to FOSS. You don't get it from Android.