Please, Help Me Understand

Hello everybody; I'm trying very hard not to sound salty right now, but honestly League has been giving me a headache recently. As a player, I personally prefer to play norms once I've hit a certain rank (plat, here), and see little point to ranked in the preseason. However, I feel like my quality of games in norms is so horrible! For example, if you check my most recent game, you'll see my team had a plat, three golds, and a silver, whilst the entire enemy team was mid plat. Now, I understand that there is a mmr difference in ranked and norms, but even then... the difference in play was verrrry evident in my team. So, can anyone people educate me on why matchmaking in normals works this way, like, a logical reason? Again, not trying to be salty, I just want to understand. Thanks

[zombie-nunu-bummed]