In World of Warcraft if you don't have certain achievements you will be denied invitations to groups. They are used to determine who is good enough to party with.
Here is a list of my achievements: http://www.wowarmory.com/character-achievements.xml?r=Feathermoon&cn=Eratosthenes&gn=Malice
They're good enough to receive an invitation to anything I want to do. Its my personal in-game resume. However, if I didn't have those, I would have a very very difficult time finding players to invite me or won't decline my own invites.
Do you want to bother with maintaining an in-game resume of your skill and ability? Its a chore.
You are talking about pugs. Raiding in WoW is ludicriously easy, but does require teamwork. If you dont't have teamwork (AKA a pug), it requires experience and overgearing.
If you do not overgear a raid, and do not have experience in it, no pug would want to invite you for a raid that would take 3-5 hours, even with a compitent group. No pugged group is going to spend 30 hours a week learning how to clear a Raid. That is what guilds are for.
If there is a person who has no raiding achievements, they are either...
A. Fresh level 80 who is compitent, and hasn't yet raided with their guild.
B. A level 80 person who is beyond incompitent, and thus no guild has allowed them to raid.
Is it any surprise that people in that game treat achievements like a resume when inviting COMPLETE STRANGERS to complete content that is only supposed to be completed by a very compitent group?