It's because they think they're math geniuses.
They think it's the World Series of Poker where if you don't have a 89% chance of winning you fold the hand and wait for the next one. What I don't know is how these people think they know the math behind what their "percentage chance" is of winning just because bot lane gave up first blood. Even in a worst case scenario where you only have an 11% chance of winning, you have a 0% chance of winning every game you surrender. Anyone who doesn't want to wait for a win is just selfish, but then again, that's the real reason isn't it?
The people who surrender early all think that they're the only person on a 5 man team who can win the game for you (especially if they're the ADC, the mid laner, or Riven.) Once any champion on the enemy team gets ahead of them, they give up because mathematically they don't win automatically. The thing they fail to understand is that even if you lose mathematically, teamwork, outplays and macro strategies still exist and can still get you a "net" win.
These people don't care about this, they just want kills, they don't want to "play" the game with actual skill, they believe that when champions clash in this game, mistakes are never made, misplays don't happen, outplays don't happen, strats that revolve around anything other than getting kills don't matter, and the team that is mathematically ahead wins 100% of the time. But that simply isn't true. Just because something happens most of the time, doesn't mean it happens every time. In fact, the further away you are from pro-level play, the more true what I've just said becomes too.