I was doing some reading and apparently on Kinguin they have genuine keys that come from places that buy them in massive bulk and cant use them anymore or something, according to the person that owns the Kinguin website.
Because it's a better explanation than the truth, which is that this is a common way for people to launder money. Steal a credit card, buy a bunch of keys, sell off the keys at a discount. No physical products to deal with or to fence. And even in the best case scenario, companies do not have the right to resell "extra" bulk keys in this manner. Your key can be revoked at any time by Microsoft.
In other cases, key resellers have caused a lot of hardship in software access for third-world countries. Companies used to be more willing to sell software at a significant discount in poorer countries, where sometimes software prices can be the average monthly wage of someone. But key resellers exploiting this has caused companies to be less willing to do this. Aiding key resellers helps exploit poverty.
You can run Windows 10 without a license perfectly legally and you just lose a few customization options and have a watermark (and yes, you get all the security updates).