The chargeback thing, plus most games are fiddly and complicated to pirate these days. (And many multiplayer games can't be pirated at all if you want to play online.) IMO, the #1 reason for the decline of PC game piracy is that Steam makes it so much easier to just buy the game.

But buying a G2A code is almost as easy as buying from Steam. And a lot of people who wouldn't pirate for ethical reasons don't realize it's not legit. So G2A turns loses you sales from people who would otherwise have bought the game legitimately.