This is for the longevity of the game, to make people play both classic and retail. You don’t have to make the items the exact same appearance but something that will make retail players go to classic for an extended period and something that will make those who play classic try retail.
The “people will just do it for appearances/mounts/pokemon” argument is sophistry.
Imagine someone joins a mythic raiding guild with the intent of just getting the mount from the last boss of the tier. They still have to prepare for raids, integrate into the team and follow the raid mechanics, they won’t just /gquit after they get the mount either. How is this any different from that?