Two main reasons imo:
- People don't realize that quitting just ruins a game.
- People know the above but believe the game is already forgone and that you idiots are too stupid to realize it and keep wasting their time by not surrendering. That's not me btw, that's just the thought-process.
Riot can help with the first very easily. I also think Riot can help with the second by including voice chat for easy communication, but honestly it's up to people to encourage each other and explain themselves in why they think we can win if we are down 5 towers and two inhibitors and can't risk warding outside our base to catch people.
But that's my more cynical hypothesis. My more conservative hypothesis just rebuffs yours by saying, "No, people leave in league just as much if not less than other online games." My reasoning draws from LFG groups, LFR groups, pretty much every MMO I've played, the way FPS shooters matchmaking and game design works, what is acceptable in a community, and other stuff.
My favorite example is TF2. Rounds might only last 5-10 minutes, but a GAME in A/D is until one team wins 3 times in a row on offense or until defense wins once. People leave in the middle of games in TF2 all the time. People leave a variety of games in the heat of battle. It's kind of rude to leave a LFG pug as the tank is pulling, but you get replacements so fast and dungeons are so easy that it doesn't really matter.
So with that context, why do people leave? In my opinion it just seems like they're tired of playing and want to do something else.
So now apply that to league. Suddenly, it's not that they are mad or ragey. It's just that they don't want to play the game anymore for whatever reason and just leave. And with the way that League snowballs and takes time to play out, you can't exactly blame them. TF2 snowballs extremely quickly, but then the round is over in like 30 seconds, a minute tops, unless you can recover. Some one snowballs to the extent that you can't stop them in League? You still have to wait through 5-15 minutes if you don't surrender, not because they are necessarily drawing it out but because the mechanical limitations of the game prevent them from ending it in 30 seconds that early.