To be fair, and keep in mind this really only applies to low elos (bronze-silver and sometimes even low gold), throws happen extremely often. I've won game where we are 5-32.
Hell, even recently I won a game where every lane fed, simply because we could hold them long enough for myself to get farmed (Vayne adc) and get us into the late game. we didn't have any nexus towers and had taken non of their towers and drags were our 1 to their 3 and we were still able to win because low elos don't know how to close games.
There's also the fact that you have to look at team comps. Does your team have a lot of waveclear to be able to stall out the game enough for you to catch up? Do you have defensive tanks (not dive-orientated tanks) to help against tower dives? Do you have a team that scaled better into late game than they do? A lot of times it's easy to win even from behind simply because
- You have the late game
- Enemy doesn't know how to close games
Now I'm not saying you should never surrender, I'm just saying it's not always a hopeless cause simply cuz you got roflstomped in the first 20 minutes of the game.