The steam key is used for verification of the purchase and adding the game to the user’s library. Some games on steam do not have DRM and will continue working after the game gets removed from the user’s library.

With or without DRM, and whether or not the platform/devs spend the time to remove fraudulent purchases from user’s libraries: if the game gets first bought with stolen cc info, then someone will after a few weeks have to deal with the chargeback administration and pay the chargeback fees.

Afaik, least work and least unpredictable costs for the Devs would be to sell only on platforms where the platform pays the chargeback fees and handles removing the game from user libraries (or not). Which would mean that the game is only available through large platforms like steam/epic/gog, which take a large % cut.