"EVERYONE". There is your problem.
The team that's doing poorly has 5 people in it and if even ONE of them tilts or starts raging their chances drop so hard. The team that is winning can have this problem too, of course... but it's FAR more common for people to start bitching, flaming each other or trying to piss blame at each other on the team that's behind. It snowballs from there.
For example... The top laner dies to an early gank and rages at his own jungler... Because he's tilted, or because he's an idiot or even because he's RIGHT and the jungler was farming blue when he got ganked and could have easily turned the gank around if he'd come in to help. It doesn't matter why he's doing it, because he's raging at his jungler now. The jungler is pissed off at him now, justified complaint or not, and stops ganking for him. Bot then asks for a gank because THEY are struggling. The jungler is pissed at the top laner for demanding ganks, so he assumes bot is being pissy at him too... or he tries but due to being behind, he fails to get anything and a laner dies. Now top is raging at him harder and calling him inept. Or he's salty that he "wasted his time" instead of power farming and in a bad mood because of the top laner, so he blames bot for doing something wrong. Or he just plain stops ganking entirely... leaving the whole team out to try. Now every lane is failing and the top laner is raging at the jungler and because the jungler isn't helping them, mid and bot jump on the jungler too. Everyone is too busy fighting over who is at fault that they all play like crap and then they lose. They're not even really "playing" at that point, they're just arguing over who's fault the loss that they have ALREADY DECIDED WILL HAPPEN is going to be.
This happens all the time and it's a huge reason why comebacks are rare. Because teams rarely stay calm, focused and collected when losing. Teams rage, and blame, and tilt, and give up. People afk, or just stop trying their best... or actively play stupidly because they don't care anymore. Even teams that don't rage tend to write games off as losses if they're going badly and stop trying as hard to win... because it's stressful to give it your all and lose. So even calm players will pull back from the game before it's over and wave it off. Remove themselves from it for their own emotional well-being. It's... just a thing that happens.
Comebacks happen... but god DAMN are they rare. Mostly due to simple human nature and the fact that the community is full to the brim of whiny, immature people that prefer crying and fighting over who is at fault for the problem to actually FIXING the problem.