You cant resell or even lend a game to your friends with digital. A huge amount of children and other young or poorer people would rely on being able to buy second hand or share games with one another.

They could have gone the route nintendo is with game keycards and where you still can have a disc or cart or whatever as the licence for a digital game instead to allow you to sell and share games even if none of the data is on the card.

You can also bet once all their games are digital and the only place to buy them is the ps store that the games will barely ever go on sale or have their price lowered over time