Well sometimes people are forced to surrender. When one surrenders it's not leaving the match, and there's no long term penalty. For example, let's say you're in a game with a teammate who's doing AMAZING, but is suddenly being forced away from his computer for some various reasons, like getting kicked off without being allowed to finish, game connection suddenly fails, computer over-heating/getting fried, the plug coming loose/getting yanked out of the socket.
Or, on a flip-side. say you have a great teammate, but they are being forced into being a detrimental factor on your team. For example, they could end up being hit by a severe glitch, like I once was, and it made me unable to see minions or neutral monsters. Since I was Veigar, my ability to become stronger was extremely hindered, since I was unable to see most of the enemies (I was still taking damage from them, but I also got the amusing side effect of not having unit collision.), I couldn't hit them with any targeted attacks and spells. My only way of damaging them was AoE damage. Ergo, Dark Matter. That match was my first surrender ever. I was literally unable to help, no matter how much I wanted to. I did it for the sake of my team.
To keep my statement going, your teammate could also be hit hard by lag, rendering his actions delayed greatly, or making him unable to see any of the action when he needs to, as in a lock up.
To simplify my statement, surrendering is kind of a "In case of emergency, break glass" feature. You don't have any intention of using it at most any time, but when it's needed, you have almost no choice.
But yeah, about the games not being over until they're over, that's how I play too. Even when I'm losing ludicrously. I ended up winning a game for my team as Master Yi where the scores were around 38 - sixtysomething, the enemy team was fed, and we only had one turret left.