It's because the game is trying to force you towards a 50% winrate, so they start stacking the odds against you. It happens with winstreaks, win 5 games in a row and I fucking guarantee 1-2 of your teammates in that 6th game will be on a significant losing streak, like 8 games. It's the system trying to balance binary aspects, extreme winners with extreme losers to create a mediocre result.
It doesn't feel fair but from a mathematical standpoint it makes sense. If you continue to win those stacked games, then your mmr is increasingly inflated and you receive more and more stacked games while your mmr is inflating. LP gains have a cap, but ultimately it will result in you skipping multiple divisions until your gain of LP finally meets your gain of mmr.
The problem with this system though is that your MMR inflation and the stacking of the team doesn't really account for the fact that the game is team based. An elo system, which originally was used for chess, merely matched you with higher rated opponents until you eventually get a 50% winrate. Unfortunately, the way that your mmr inflates is still trying to treat you as if this is a 1v1 game, it's not, it's a team game, so by matching you with teammates that are extreme lose streaks you tend to receive trolls or boosted players exceedingly often if you are climbing, particularly if you are smurfing, so the true measure by which you have a game stacked against you cannot be a true measure of your skill because your teammates may keep you down in a situation where you otherwise would win. Essentially, eventually you will be handed a game that is quite literally, impossible for you to win.
Games where you see the enemy team having multiple mid diamond players vs low platinum players occurs when one or two of the players on the ally team having ridiculous mmr and a winning streak, that's forcing the system to warp matchmaking around them. That's why as master tier players reach gold and platinum their climb significantly slows down (gold if from b5, platinum if from unranked) because the game is trying to FORCE them to lose more games so they don't have an 89% winrate.
TLDR: Games are stacked more and more as you win while on a winstreak, so that smurfs climb way faster and players who aren't smurfs will settle back down. Feels shitty but it makes sense mathematically because of the ELO system league uses.