It's a bit weird to have achievements, imagine you and your group of friends at school or somewhere, you don't really do stuff like make achievement lists like "hangout 5 times" or something.
We are a group of friends in that sense xD
Though I understand your point of thinking, I used to think the same for clans and that's why all my previous clans were rejected, because they were treated as some sort of club that we had to work at and motivate people with odd things to stay in the clan.
Whereas simply put a clan is meant to be a group of friends and that's why everything is simple and we all have mutual interests in games and anime etc.