There are several those keys can be obtained, and most of them don't involve fraud:
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Purchasing keys in a region where they're cheaper, and reselling them in regions where the game is more expensive
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Purchasing keys during a sale, and reselling them after the sale
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Claiming keys from giveaways and selling those when the giveaway is over
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Buying a bundle (such as Humble Bundle) and selling the keys you aren't interested in or you already have
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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.
The main reasons why it's Doom specifically are also because:
The game is open-source: https://github.com/id-Software/DOOM/pulls?q=is%3Aopen+is%3Apr This makes it much more doable to port it to other platforms (and to strip out anything not absolutely required to get the first level to run when you run into technological limitations) than when you have to rely on unofficial modding tools.
It's nearly 30 years old and designed for computers with only a few megabytes of memory and for processors of well under 100MHz, which are specs which the majority of modern systems have, even embedded systems. It also renders fully on the CPU and doesn't require specific hardware like a GPU or a specific graphics chip.
Being a first person shooter with 3D-ish visuals it looks a lot more impressive than if you show off a simple game like Pong orTetris or something like that. It has the right balance between performance requirements and impressiveness, and it's also a game that was very popular in its time and it's instantly recognisable to a lot of people.