I think every player that played HON or Dota2 knows how
beside the obvious answer "HU TURN RATE" here is how it works in those games:
-creeps have more base AD or as much base AD than ranged champions (who have around 40 AD at lvl 1)
in league, creeps have like 10 or 20 AD i don't remember, while ranged champions can begin the game with 70 AD thanks to AD runes and doran blade. In Dota you can't get +8 AD from an item.
creeps have also WAY more base movement speed, they rune so fast if you continue to poke someone for a long time you will lose x2 more hp than the poke. In league creeps start with 325 ms while ranged champions have between 325 and 350 ms depending on runes/base ms/if the adc gets tier 1 boots after recalling
The combination of all of this makes marksmen able to attack you permanently in lane i can record a game where enemy caitlyn just stood there between minions and continued to poke me as support darius
-Melee also have the option to buy a decent item protecting them, dota2 for example has stout shield which cost 250 gold gives no stat and block a huge amount of damage if you're melee, in league doran shield is a joke and doesn't make you survive against ranged champions
with those 2 changes you already improved by a billion time the match up melee vs ranged in 2v2