Lp and series

Why are there no lp gains on won series tha are not tier advancement ones? I mean if you lose series your lp loss/gain (considering the case of 1/2 w/l) per game lost is cumulative and gets substracted from your total 100 lp, for example you get dropped down to 60 lp after a 0-2 series and around 80-90 for a 1-2 series.Why does that not happen when series are won ? It's not like advancement from div 4 to 3 would give an unfair advantage since it isnt of much significance so why not start with 20-40 lp ?

tl dr

Lp per win on series should be gained at the new tier instead of starting at 0