I Only Listen To Underground Metal

Thats a statement I am so tired of hearing. Like it makes you that much cooler than everyone else even when you know it’s a damn lie. What makes “Underground” metal better than mainstream?
I don’t completely understand what the statement “I only listen to underground metal” make people think they are better than every other metal head because they listen to a bunch of bands that haven’t been and possibly don’t want to be signed to a record label.