Why does placement system punish ppl winning

I was last season silver 3 and got to silver 1 preseason so my mmr isnt that low. (got bronze 4 from first game) I have played 9 of 10 placement games and have won all of them and after every win my lp gain gets lower it started from 50-60 and from my last win i got 32 and game bfr that 34. So why is it that the ppl who is winning are getting less lp after every win and bcz of that im not even getting out of bronze now from what it seems i will finish 90-95 lp bronze. (Im playing with last season golds and silvers) Im playing with my friend and its happening to him too.
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