To put it into the comparison, it would look like this:
In CS GO, If your team is better than the enemy team you will eventually win the game unless It is already 11-4/12-3 for the enemy team but even then, you can still come back and win If actually play better than the enemy team, It's like getting a bad grade in school, you got a bad grade (lost round) but you can always study for the next exam (play better next round).
In league, If enemies are fed It means that not only that you'll have to play better If you want to win, but you'll have to play MUCH better, mistakes will always cost you more and in the later part of the game, one bad play from your team can easily decide whether you lost the game or not.
To make an analogy:
Losing in league is like a train going downhill into the abyss, If you let the things go on their own way you will just end up losing almost 100% unless enemy does something stupid, If you are losing your whole team gotta make a play or be close to the temmate that is making a play and act accordingly to that play.
One full HP champion will never win against 5 enemies, you gotta be giga fed for in order to win that (and that won't be more than 2% of ur games), doing 1vx (that is 3 and above) in lol is near impossible If not impossible most of the times, which is not the case in CS.