Piracy

Yes, only the publisher/developer can generate keys. That's where all key sites get their keys from. You cannot obtain them through the Steam store. And that's also why games that simply do not sell keys to wholesalers don't have this issue. Just look at Factorio. They don't do sales, they don't do bundles, and they don't sell keys. You can get it on key sites but only as a gift and for the same price it's being listed on for on Steam.