Probably because of the fact that you can't make plays on a team that makes no mistakes.
But the enemy team will make you look stupid when they capitalize on mistakes that cost you more and more as the game goes on longer and the death timers start to get ridiculous.
I don't have any solutions for this, but the fact of the matter is games are won or lost in League of Legends based on which team played worse overall, not which team played better and capitalized. Comebacks are infrequent and usually not worth the time and effort burnt trying to obtain them when you're out of Silver since you don't learn anything new.
The fact that you can't readily backdoor, and that Flash exists is actually one of many little things that caused this, a weaker team can't readily capitalize on a stronger team's mistakes by ganking a critical teammate that is split pushing because they will flash out of the gank or the main force of the stronger team will have pushed down two towers by the time the split pusher has been disabled. That's only one example of many, others are Zhonyas becoming a mage crutch so that mages can make suicidal plays and get away with it. In fact Morgana, Fiddle, Kennen, Lissandra, and potentially Karthus are all designed around exactly this. This is teamfight stuff, you can't simply chalk it up to duelling.
League also has a nasty habit of forcing people to split push all 3 lanes slowly to solidify their advantage rather than ending the game as quick as possible. This doesn't really allow for as many turnarounds as a game should have, if you want to turn a game around against most stronger comps, they have to be pushing into the base 5 man in order for you to force a fight on bad positioning underneath your tower or inside your walls. If the jungle geography just makes split pushing the most efficient option, then ending the game quickly is far less of a priority than doing so safely with a moderate gold lead and high mobility.
By contrast DOTA2 can and does let you win or lose based on which team plays better at a specific moment rather than how their overall play was to that point. Gold leads don't matter if the right champion can make a play based off the right opening. League champs are too generalized and simplified to allow this in most cases.
