There aren't enough players, it would take too long to find games. Relic's algorithm is likely smart enough to make sure the two random players have a higher average rating than the team in order to offset some of the advantage that comes with better communication.
"Take too long to find games" is a really tired argument. While I obviously can't speak for everyone, I for one would rather wait 15 minutes for a good game than stomp a newbie after 3.
Also I wouldn't put much faith in Relic's algorithms when their handling of the new provisional ranking is obviously dumb as a brick (instant...and completely random matchups for 10 games in each game mode for everyone, and with each partner too in AT). For that matter, looking at what kinds of matches higher ranked ATs seem to get, I seriously doubt the matcher is doing anything except try to find players with a combined elo at most x percentage away from the AT, where x keeps growing until a match is made.