That's not entirely true. Often you are playing online with players you may not know, and the game has mechanics for stealing from other players even.
It's not a 100% co-op game, but even if it were being able to let everyone see and have a fair shot at the loot can be helpful sometimes.
It is 100% Co-op. They can mess you over, but they are only hurting themselves in the short and long run.
In a MMO you can just do a different raid, join another group, do something else.
