Probably a perceived "physical version tax". You could also just buy the PS5 version while it's on sale for $14 and you'll get the PS4 version, too (confirmed).
Thanks, I looked it up and now its says the ps4 version is unavailable. I don't even know if it is still included with the ps5 version at this point for new purchases. Maybe it was a mistake. The whole thing is odd. I'll just wait on the physical to drop in price to be safe.