You wanna know why I really surrender?
Actually, there's 2 reasons. My team has shown that they don't have the skills to turn the game around (at least in the first 20 minutes I've seen them). This is seriously important and I think people don't see this. If my team just sits there whilst I'm trying to force them to compete for vision, advantageous skirmishes and objectives, I'm going to lose hope very fast. Overly passive play doesn't win games, and if you're behind, it only slows down the snowball, it doesn't actually stop it.
The second reason is because I'd rather go into a new game, and learn how to stop a Katarina from going 15/0 when she's 2/0 rather than trying to learn how to shutdown a 15/0 Katarina in the first place.
Basically, I'd rather use the time (5-15 extra mins) in a new game learning how to snowball rather than trying to play against an opponent I have a 5% chance of beating.
Edit: Honorable mention is because higher elo players close out games MUCH cleaner than low elo players do. Bronze to Gold players are quite lazy with their roaming, vision and objective control, Plat players are too arrogant but get much better in this aspect. But once you hit Diamond, you're gonna be dragging out a lot of lost games.
The only problem is that I quit 2 seasons ago so I'm still learning the ropes again.