Well, I would assume they enjoy the journey of them.
- Achievements in themselves would be useless if they weren't a journey or somewhat difficult to "achieve"
- Pets and their battles are also something you have to progress through. Would people care as much about pet battles if they simply received every single battle pet in rare & lvl 25 as soon as they collected one of them?
- Mythic Raiding would be absolutely no fun and not enjoyable if you went in and one-shot every single boss right from the bat. You have to work towards it by progressing through boss encounters.
It is very difficult for me to understand this attitude of "I want everything now without putting any effort into it because ... well because I want it". Don't you guys think like 2 meters ahead and realize that if you got that, then the game would be basically over immediately?
Maybe I'm old-fashioned, but I'm a firm believer in "If you want something, work for it".