Here's why games are one sided:
It's human nature to focus on the one thing that isn't part of the plan. Your plan is to play safe, wait for opportunities, and get kills/objectives, and possibly to carry your team.
In reality..... you play well for 5 minutes, your team suddenly gives a First Blood, and instead of focusing on your lane, you begin to wonder why/how. You begin to miss CS, get poked when you shouldn't and pushing out further than you should.
Your game plan just went to crap, and you begin to externalize on your team. It's all their fault. It's them who got you killed. It's them who fed first. It's them who played something off meta.
Yup. All them. In the meantime, your cs is now 30 behind, your level 2 behind, and you are not warding properly. You're 0/2, and suddenly started to feel that 0/2 can become 4/2 if you just focus. So you begin to yolo, trying to get those kills without realizing you already gave your opponent a 2000 gold lead, and they can obliterate you even if you land everything perfectly.
0/3.
This is happening to every player on your team except the one who gave the first blood.
They're doing worse now, because the other 4 who are going through this thought process, suddenly got loud, and accused them of everything in the book. No one likes to be accused. So then, ofc, they just don't care. Their team are angry toxic mofos, and the game is already a loss, so why not afk farm, or just int feed the rest of the game?
If you can buck the trend of this mindset, 1 sided games will diminish. It may end up being a 2v5 or 3v5, but the players who get away from thinking like this even half a game...... are the ones who win more.
For example:
https://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2999525618/243999882?tab=overview
My team didn't give a damn about doing their jobs, because I Morgana Adc'd. Off meta pick. Instant rage.
I spent an entire lane with a Brand support...... pretty much 1v2. Brand was pretty much inting to Malphite ults/MF ults without throwing a single skill.
As a consequence, I fell behind on the ability to farm, but I didn't feed....
But look at Irelia and I. Our entire team was feeding, blaming the adc Morg, but Irelia and I began just trucking along and setting up clears.
Eventually after a couple clears, we ended game with 3 pretty much raging int feeders.
https://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2999580592/243999882?tab=overview
A little less noticeable, but I actually gave second kill to Kha. He was invading my jungle. The pressure got my team raging a bit, because Voli needed help, and Zed was vs a Urgot. But I needed what farm I could get, and I only took sure thing ganks. I had to block Voli/Zed because of their raging over slow ganks/pre-eminent Kha.
But..... look how it ended. I murdered Kha the entire game. When he tried to side-bush bot lane, I was there. I slammed him and he had no way out. I solo'd fiddle 4 times. Entire game, I was 100-0ing the other team, even with the criticism. After I got ahead, I was just omnipresent in keeping my team from feeding Urgot.
This game became so one sided against the favored team, they surrendered to me.