I have to disagree with this.
I like to go trough some game and get everything.
Zelda is one of the games I enjoy 100%ing. I feel like it's a challenge to get 100%. They left you other options. You can simply ignore those unneeded side quests and win the game without the.
Pokemon, I've never gotten 100%, but every time I play pokemon, I try to get as many as I can. I simply enjoy doing that. I HAVE fun. Same as someone has fun (again me) using a Sniper rifle in a shooter game and kill with ONLY headshots, some other people have fun collecting things, finding hidden stuff, catching all pokemon or getting the Aqua Sword in PoP WW. I DON'T need a reward. My only reward is that I know what I did. I enjoy proving to my self that I CAN do it. The best way to prove that is to.... yup, you got it right, to do it.
Sorry Yahtzee, but I'm disappointed. Just because you and some people you know don't enjoy it, doesn't mean no one has fun. Fun is maybe the most subjective word and you simply CAN'T generalize it.