This isn't Chess and league players aren't master chess players.
Being a team game designed the way league is, while it does have standard elements of strategy and tactics, it is by no means on par with something like chess or how chess is designed as a game.
In chess, there are only so many moves you make until there is a winner (or loser); with league, willpower and the ability to work in unity and focus to achieve objectives, is just as -- if not more -- important as strategic thinking and gameplay mechanics.
There is no pre-defined event outside of your nexus blowing up or having someone afk, that determines your victory or loss before your nexus blows up. Objectives are almost always achievable, and it is possible to make comebacks through a combination of sheer willpower, determination, itemization, teamwork, focus and a lot of luck.