There are several those keys can be obtained, and most of them don't involve fraud:
Purchasing keys in a region where they're cheaper, and reselling them in regions where the game is more expensive
Purchasing keys during a sale, and reselling them after the sale
Claiming keys from giveaways and selling those when the giveaway is over
Buying a bundle (such as Humble Bundle) and selling the keys you aren't interested in or you already have
Buying games with stolen credit card and reselling those keys
Only the last one is illegal and costs the developers money. Digital storefronts have made it harder to obtain raw, transferrable keys and have introduced region locks to try to combat those top 3 methods, but they all were very common in the past.
Key resellers like G2A are pretty much just an eBay for keys. It's not an illegal organisation, they just provide platforms on which people can sell their game keys, but they don't know (and probably don't care) how those keys are actually obtained. The majority of keys on those platforms are actually legit (iirc by far the biggest category is games purchased out of region).
HOWEVER,
The legally obtained keys sold on the platform are all obtained in such a way that the developers get little to no money from it, so chargeback fees from a few fraudulent purchases easily outweighs the small amount of money they get from the legit keys there. So even though the majority of keys sold on such platforms are not illegal, the few illegal keys that do exist are enough to make the developers still lose money on average with keys sold there.