Why mouse-1/2/3 ?

[ndame]

I don't use the mouse in emacs at all, but when browsing info mouse-2 caught my eye.

Why emacs calls these mouse-1, 2 and 3? Wouldn't it be more user friendly to say

left mouse button, middle mouse button and right mouse button?

I checked what terminology Gnome uses:

"Press the right mouse button on any local folder"

https://help.gnome.org/users/shares-admin/stable/tool-getting-started.html.en_GB