Why do you lose strip poker when you're naked, that's stupid, I want to undress. It's the same with drinking games. Why does the loser get to drink and not the winner when the point is to get drunk?
Well being naked is embarrassing, so I would think most people don't really want to undress in front of others.
Regarding drinking games: it's kinda like "I lose the game, but at least I get to drink!". This way everyone has fun even if you're losing. The winners can still drink even if they win anyways, it's just not mandatory like for the losers.
Oh, then my group of friends just played them wrong. Winners only got to drink after, not during the game, but the way you explain it makes so much more sense.
I played tetris against my friend once, and she was still learning the game so I said I'd take a shot everytime I win so that it puts myself at a handicap.