The reason people don't surrender is because in League of Legends throws are super common. People in the lower elos especially do not play with their lead well and tend to get cocky. The amount of times I have been down 20 to 3 and the enemy team instead of pushing objectives and map pressure seeks kills is countless. The only time I surrender is when I am down two inhibs very very early in the game and the enemy team is working cohesively as a unit to choke us out of our jungle/baron/dragon so we have no opportunity to come back.
being down in kills, objectives, etc, means nothing if they aren't pushing that advantage to choke you out from coming back. If they are simply roaming the map to get more kills to up their KDA that is your chance to win.
Sure, a lot of those if not most snowballed games are nearly impossible to come back from...but let me tell you there is more of a chance than one would expect. All it takes is two won team fights or a snuck baron and you are back in the game in league of legends. and if their team isn't working together, warding, is splitting up trying to 1 v 3...etc....you can snowball the game back in your favor.