I used to think this, but actually, it's a bad idea. It's easier to pick when you start a game than when you end one, so if points were awarded on the basis of your rating at the start of the game, you could inflate your rating by starting a lot of games when it was low.
Also, a game can take many weeks (or months) and it is much more fair for the result to be assessed on the players' current rating than one which is months out of date. This is particulary true of people who have recently joined the site: I lost a game to CurtCastle which we started when he was rated p1200 and finished when he was rated 1435. Since he is now rated 1627, I am sure it was fairer to me for the points to be awarded based on the ratings at the end of the game.