Why is Plat full of trolls?

I went from Silver 5 to Plat 5 this year. Failed my promos 5 times to plat 4 recently, I keep getting trolls/afks/toxic/just plain bad players on my team that I don't even know how they got to plat in the first place. I want to improve and its only fun when I actually have a serious game, not someone building tear on lucian first and feeding or 4 zeal leblanc. I came into plat thinking finally its all over, I will be with people who want to win and get to diamond but thats all a lie. These plat 5 should be in gold 5 and there are some who were plat last year and are still plat 5 which is really sad. This is really affecting me and I'm tired of all the trolls, I get them in almost every game and it isn't fair. Its making me have a bad mmr and losing 20-22 points a game even though its not my fault, its out of my control and there's nothing I can do about it. Gold 1- Gold 5 was so easy but plat just act like a bunch of silvers and play like them too its really sad and it bothers me. I thought this was the beginning of what is considered "higher elo" then bronze-gold's "low elo"