Why do champions with the highest ranges and most mobility also egt acces to more of it from items
every
upgrade

In addition to gaining full of near full benefits from items like
& 
then there is also the issue of the flatpen items like youmou's & Duckblade giving a lot of mobility to both adc's and mobile assassins alike.
I remember boot upgrades being removed because they overly favored mobility updrades, and along with other options gave too much mobility in general.
I think the excess of mobility is a problem for a lot of immobile and low mobility melee champions
When ranged champion not only have the range advantage but also a mobility and cc advantage things get a bit imbalanced. (And often a damage advantage too looking at you marksmen)
I really wish we could trim all this mobility down, because it's mostly accessible by the champions who need it the least, they already have mobility spells or big range advantages, and now they can simply outwalk and permaslow people too It's kind of gross