It's not necessarily a "defeatist attitude", it's a calculated decision that people (including myself) often make after weighing several factors and making a prediction about the outcome of the game.
After playing league of legends for several years... or even just months, you get an understanding of how a game is going and where it is headed at any given time.
Let's talk theoretical: You've been playing like crap, fed the enemy team several times, you're team has played like crap, fed several times, never wards or gets vision before doing anything, etc. You have lost all of your outer turrets and they have lost none, the enemy team has late game champs and you don't, you're team isn't keen to team-fight or even play at a decent enough level to play safe and not feed anymore regardless of what you have tried to tell them.
At this point in a game, it's a calculated decision to take the loss early so that you can use that time you would've spent on a game that is obviously not going to be won, to work on and play a game that you have a much higher possibility of winning. Of course you COULD win, anything is possible, but chances are... no. Especially in low elo when like I said before, you've played out these scenarios many times, and you already know based on the way your teammates are playing and the enemy team is coordinating against you, that you are simply outclassed/beaten. It really becomes an annoying waste of time when you are trying to climb.
In my opinion, some people may have a defeatist attitude, sure, but in most cases, players are simply making a calculated decision to move on from a loss to a possible victory. I understand the whole "never give up, never surrender" type of attitude, but when it comes to gaming, it really doesn't apply in the same way as it does in real life. There are no lives at stake, it's a game, and you lose sometimes regardless of your attitude. Maybe you've won a couple more games because you never surrender, maybe I've won a couple more because I surrender early and get more games in, different strokes for different folks.
TL/DR -It's a calculated decision to take the loss early, rather than late.