If rating would be calculated from as it were when the game started I would do something like this:
(1) Start a great number of games, and after three moves resign them all. My rating will go down to, say 500 points.
(2) Start another great number of games, hundreds, perhaps thousands of games. As everyone of these games would be calculated as if my rating would be 500 points, my rating would sky rocket up to 3000, 4000 or even 5000 points in very short period of time.
This wouldn't be fair. I'm glad that the rating is calculated as it is when the game is ended. As it currently is.