The average is more like 25-30 minutes. If it breaks 45 minutes then either it's a really even game (which is fun), or the team that has the lead doesn't know how to use their advantage to win (which is excruciating for both teams).

Still, you're right, that's a long time to be locked in with someone who is clearly weighing down the team. And of course, that person is probably embarrassed about their performance, so they'll often lash out at other people to deflect from themselves, which will tilt the rest of the team even if they were chill until then.

The advantage that a lead gives you is also a lot bigger than in most other games. Players get drastically more powerful as they gain XP and money, so if one team gets an early lead, no amount of mechanical skill will be able to counter their advantage. The only way to catch up is to avoid fighting, drag the game out, farm NPCs, and try to punish the enemy's mistakes. That's extremely frustrating if you're not the one who died and gave them the lead.

It's hard to avoid getting caught up in all of that unless you decide to focus completely on your own game and improving over time, and stop caring about individual wins and losses. Most people don't have that kind of perspective. I didn't either back when I played League a lot.