Why does riot do placements?

I started playing league in season 1, ranked in season 2, and every year i dread the new season. Every season brings new changes, witch are usually great, but it also brings with it the worst thing riot does, in my opinion. Placement matches are the worst. I always try to be optimistic, I always think that if I just win most of my placement matches I'll only get demoted 1 or 2 tiers. But every year without fail I try so hard on my placements and usually win most of them. Last year I won 8, this year 7, but I always get demoted so far down. This year I won 7 and got demoted 4 tiers from Silver 4 to bronze 2. While this doesn't seem like much when people in diamond go down to plat to me I work hard and do my best for every lp and tier i get. It inst a lot but I would like to keep it. So my question is Why! Why do we get punished just for it being a new season. Why do I have to fight my way back to where I belong through trolls and afk's. Every season I contemplate quiting this game because it feels like no matter how well I do it doesn't matter i just get reset when the new season come around. There has to be a better way to reset our MMR without screwing us over.

TL;DR: FUCK PLACEMENT MATCHES! PLS RITO SCRAP THEM!