If you can survive to lategame, it is sometimes possible that a team with a huge lead will throw somehow and be dead long enough for you to push their base in.
But if this happens in 5% of games where you are losing badly (and it's probably fewer), is it worth adding an extra 10-15 minutes to every single game that you're 95% certain to lose at 20 minutes?
Playing more games means you can win more games. If you have a positive win percentage, then playing more games means you will climb faster. If you only have an hour to play, it is not fun to have two people insist on dragging out what is clearly a loss at 7 minutes into a 45 minute nightmare.
And, frankly, even when likely losses turn into wins, they're rarely the kind of wins I feel good about. Usually it is something like Susan farming stacks in top lane for half an hour while everyone else gets kicked in the teeth, and then he wanders mid and aces the opponent.