This is about removing any and all ownership rights of buyers.

Not just of selling you copy second hand but also things like lending (even to your own family), gifting and even playing it in a different device

Mandatory Age Checks on the device also helps with stopying lending even to members of your family that live with you and use the same device to play the game: if the hardware (with the excuse of Age Checking) identifies the user, then the copy can be bound to a specific person in a specific devive rather than the device alone, so for example two siblings in the same household need to buy two copies to play it even at different times rather than just one copy as the “age check” identifies the user and only allows a specific user for a specific copy.

The fewer rights buyers de facto have, the more copies publishers sell as users can’t get the games by alternative legal means, and whilst at first there might be some backlash from taking those rights away, in my experience not only are most gamers sheeple or at best dogs that bark a lot but don’t bite, but usually they over time get used to not having those rights (especially as new, younger people, who grew up without having had them become gamers), so pretty much all backlash from taking those rights away eventually dissapears and those companies end up making more money than before.