It's a digital product
A key is a sequence of characters that activates online and arrives in your inbox instantly. No box, no postal service, no courier — there's literally nothing to "return" in the physical sense.
Under EU consumer-protection rules, the UK Consumer Contracts Regulations, US state-level digital-goods statutes, and equivalent national laws, activated digital goods are not eligible for return or exchange. The same rule applies to Steam, PlayStation Store, Xbox Live, Nintendo eShop, and GetAncient.
An activated key isn't a defective product. It's a service that's been delivered and used — as irreversible as a movie you've watched or a concert you've attended.