Aside from the cost of credit card charge backs, consider that these are (mostly) customers who think they are buying the game legitimately and just getting a good deal. That money is going straight to g2a instead of the devs/publishers.

Maybe a customer wouldn't buy the game at the full price, but since they're not pirating from the get-go, that shows they are a customer willing to spend money. Maybe they wouldn't buy the same game or as many games, but it's possible they'd still be spending some money on games through legal (but more expensive) means if they knew g2a was just piracy with more steps.