RP Penalty for Teammates Leaving

Why is this still a thing in these games? I was ~200 RP away from platinum, so effectively 1 win, but within the first 5 seconds of the next game, my teammates all quit, causing me to instantly lose 750 RP. Now I’m over 900 RP from platinum in the blink of an eye. Why is the person who didn’t quit also punished in these situations that are out of their control?