Riot, why do you do this?

Please inform me how going 8-2 in my matches leads me into being placed in silver 1? not only that, I STOMPED in some of my games. This is absurd. My MMR was easily gold 3 or higher and then i get placed not one, not two, but 3 tiers lower?? Wtf riot. That isn't fair. Screw this whole "you'll be placed lower than you were last season." because last season shouldn't matter if our MMR reset. if i had gone 10-0 I would have probably placed gold V or something. and that's horseshit. either this means that losing a game has FAR too drastic of an effect on your mmr in placements...or your wins don't mean shit. I don't get it. Please explain. I want literally all the intricacies provided to me in vast detail because this is dumb and it makes me want to stop playing. my MMR actually DROPPED significantly after my final match...like a solid 100 MMR lower. What the actual hell Riot? why? that's not cool. Silver players don't win 80% of their games. they win 50% at best.