Most Games are Unwinnable
That is to say, most games you play, say 70%, you cannot win. But it's still fair - 35% of games will end in a defeat no matter what you do, and 35% of games will end in victory no matter what you do. That still leaves 30% of games where you can win by playing well and lose by playing poorly - either throughout the game or in an instant all-or-nothing good or bad decision at the end of 50 minutes.
Most Teams are Uncontrollable
You just killed 3 people in the midgame and have a chance to rush baron. You ping baron and run to it, most of your team hesitates but then follows you, the adc farms and whines that it's a bad call but finally reluctantly follows, and you start baron about 10 seconds late. 5 seconds before you kill baron and make an easy escape, their team comes pouring in and aces you. The adc yells at you, "I knew it!"
This has happened to me multiple times. I felt so angry about it because it looks like I made a stupid call when I know I made the right one. But the Reality is I didn't make one call, I made two calls - and the first one was as genius as I think, and the second one was as stupid as the ADC thinks. The first call was to go for baron when we had a clear opportunity. The second call was, after the team had finally gathered at baron but a few seconds late, to start it instead of admitting defeat and letting it go.
A third of teams will know to start baron when there's a chance and do so regardless of you. A third of teams won't know to start baron and will waste the opportunity regardless of you. And a third you can control by pinging it when there's a chance. But of this third, a half will follow fast enough and you'll win the game with your call, and the other half will be a little too slow and you'll actually lose the game if you proceed instead of backing off.
The same thing applies when you know the other team is doing baron and you want to stop them. A third of the time your team just won't contest it, and a sixth of the time they'll listen to you but only too late - and in both these cases even though trying to stop them is the right team call, since your team isn't there you solo-suiciding in to stop it will actually be the worst call you can make.
And baron is just an example. Other examples are split-pushing, tower-diving, defending, or surrendering. Sometimes your team or a member of your team is just going to do or not do one of these things no matter how much you yell at them, and you just have to accept this and work with it. Notice that I didn't say win with it - sometimes the way the team is going it's just not going to be possible to win even if a win would otherwise be so easy if they just did something different.
There was a game where our nexus went down to 50hp and then we won. A member of the team was whining so hard to just give up sitting in the fountain, and another member was spending a lot of time typing back to yell at him for afking; while the rest of us just defended as hard as we could. We got a lucky pick and the two guys decided to try again and we won. Arguably the best course of action would be for the guy not to have given up in the fountain. But if we had all spent time typing at him not to do that, we would have lost, because he was just out of our control. But by defending, we had a chance to get a lucky pick and for him to change his mind. On the other hand there are games where we lose one base tower and have even kills but someone gives up and stops trying and causes us to lose when a win was much easier than it was in the 50hp game. But the fact is, a member of the team giving up is often just as uncontrollable as a member of the team being dead or not having enough items, and you just have to accept that.