I've had plenty of games where theres some salty person more or less afking who said the game was over 20 minutes ago, that we ultimately won or almost won if the person didn't give up way prematurely. (I've had games where a nasus of all champs decided the game was over at 15 minutes and dc'd. 20 minutes later they reconnected in disbelief that the game was still going, since the rest of us were still trying and managing to outplay the enemy team well enought to stop most of their advances despite 4v5. While we did eventually lose, it seems very doubtful we would have if we'd have had a nasus who'd been stacking for 20+ minutes and providing pressure.)
Sure, maybe theres a full build level 18 enemy fed out of their mind and your teams highest level is 14, but that means the worst is over and they're not getting any stronger, while your team is now consistently catching up, as painful as it may be, until eventually it doesn't matter that the enemy team is up 20 kills. If the enemy team is good, sure, they'll quickly push their lead to a victory before you can catch up, but if thats the case it'll only be a few minutes before the nexus falls so theres no real need to surrender. And if they aren't ending, then that means they aren't good, and you have a chance to catch up and get a critical pick once death timers are huge.
After 50 minutes or so, pretty much every game boils down to a 50:50, almost independent of the champion scalings, simply because of how impactful a single pick can be.