Because "Operating systems based around the Linux Kernel, excluding Android" doesn't roll of the tongue so nicely.
Everyone knows that when someone says "Linux" he means a certain category of Operating Systems.
Because Linux is an operating system. Linux can run user-space programs without any user-land.
Because the distinction is meaningless for most people.
Why do people call businessmen "suits"? Or say that "planes fly" but not that "submarines swim"? When and where does an aircraft carrier "set sail"?
What exactly is "pre-boarding" at an airport?
https://m.youtube.com/watch?v=vdPy5Ikn7dw&pp=0gcJCdgAo7VqN5t...