It’s largely retailers that drive it. As @Nugget_in_biscuit said, retailers are going to be pretty miffed if they’re selling a game above its digital price.

However, some games have found a way around this. Overwatch first came in two editions; Digital Edition and some kind of Premium edition that had a few skins. The only disc copy was the Premium edition; and it didn’t even give you much more. This was their way of discounting the Digital edition by a bit, and get more people to buy it.