It's the same as with Superheroes, it's branding.

I mean, "Superhero" is a meaningless term as it's not like the American comic industry invented the idea of giving heroic characters superpowers or anything - the Sumerians got them beat by a couple thousand years at least - but with marketing the Superhero has developed an identity of its own. Even heroes who don't possess any "super" qualities can qualify in the wider cultural sphere as Superheroes when they share the associated brand.

It's kind of like the word "apps", it's a stupid buzzword term objectively but it's been used wide and long enough within its own context - and by software & hardware developers - as to have its own meaning regardless.