I dont understand matchmaking

So Ive been playing League since season 1. Quite a few years ya know? Im no diamond payer, but im definetly pretty decent. I can fill any role I need to, I can handle "op" champs without getting over frustrated, I know map awareness, calling MIA's, when to gank, when to back off, when to fight and who to target, how to build blah blah blah. Now, over the years Ive noticed that every time I try to play a ranked game, I consistently have some really bad players on my team but the people I play against are pretty good players. My teams literally almost always start fighting with each other, leaving me sitting there like, "Guys its just a game, why dont you try and help each other?" Ive even jumped on my brothers Gold ranked account and played jst to test my theory. I do FINE on his account. I get great players who are cordial and can really play the game. Now my question is, why am I classed with such baddies? Is it because im unranked? Ive had most of my placement games but this is exactly the reason why I dont play ranked and really havent over all these years. Is it just part of the laying field? Having to crawl your way out of the bottom tier players? I think maybe RIOT needs to reconsider their matchmaking. I get stuck with folks that build zeal on Veigar,people who freak out on me for not chasing a Singed through the jungle, people who literally just DONT KNOW HOW to play the game. Why? I dont make poor team desicions, I hold my lanes, why is my individual efforts in my games not calculated into my placements? I do fine in my games but my team usually loses. Explain?