> What else could be the reason why mappers come across a "missing" tunnel tag,
Because people want it mapped "correctly" in OSM, in the sense of "consistently with other examples of similar features"?
> And why do you keep referring to a forum on Austria 
https://community.openstreetmap.org/c/help-and-support/tagging/71 is (as the link suggests) a global "tagging" sub-forum below "help-and-support" on the forum.
