Why do many people have an username like this?

Nobody can look them up on op.gg.

It's like a barcode username or something, as it's supposed to look similar to a barcode. Essentially like someone already pointed out, it's pretty much so people can't look them up, although that's kinda irrelevant now as you can just go to your profile and look at live game, or just use one of the many apps out there that tell you their stats once you get into loading screen.

Digital ski mask.

its because people used to use this in starcraft so people wouldn't know who they are and couldn't abuse holes in their strategy, in league of legends? its pointless and just for wannabe kids.

Before League it was used in Starcraft and other similar games to avoid players knowing who their opponent was, in order to get an advantage. In league this is exclusively useful for streamers. Ghosting is much harder if you can't find their name, other players that do it may be playing a mental game that maybe they are a challenger smurf, or a challenged smurf (if you know what I mean.)

I've seen 2 people/the same person twice with a name that is just a random amount of centimters and kilograms.

Why do many people ask me If my name is a reference to Tokyo Ghoul? I know why, but just find it annoying that everyone ask that.

The username is from a random nickname generator website, It just happened to sound like Ken Kaneki...