Were they AFK in the start or later into the match?
Because in league of legends, the way they implemented it was something of a "if they aren't there at the beginning of the match, you can vote for a non-match" which effectively cancels the game. However, if your teammate played and raged 10 minutes after the start, the ability to cancel the game is no longer available.
I'm going to assume your teammate left 4 or later rounds after. The reason why you can't reset is because it would be wasting the other teams time, if you always had a safe way out by having one player leave so the rest of the team doesn't suffer elo loss. You could implement more severe leaver penalties, but this could potentially affect people who simply just had computer issues and crashed.