It was the experience of the game that brought pleasure and joy, and that's what a lot of us want again. Not the numbers. When I play Classic I want to go through the deadmines with people I like and overcome a challenge together and see myself become stronger as a result. I don't want to play Classic so I can say "booyah! That number is exactly the same as it was 13 years ago!"
Most people who have spent years playing massively multiplayer online games will tell you that there's something special about the first one you played, too—and WoW was the first for many people. You can become so swept up in the uniqueness of your first experience that you overlook many flaws. It's likely many look at WoW Classic with rose-colored glasses, just as many simply forgot which features were added when. And some of today's WoW players may be too young to have even played vanilla WoW as it once was.
The fact that Cookie's Tenderizer from the Deadmines had +3 instead of +2 strength. The fact that the Stormwind south bank had one instead of two mailboxes. The fact that Jaina's Proudmore's name was "Jaina Proudmore" instead of "Jaina Proudless." Stuff like this isn't what mattered. It was arbitrary. If Cookie had dropped a shield instead of a mace and Jaina had been named Susan, nobody would have cared. It wasn't specific details like these that caused us to enjoy the game.