why does (insert champ here) fall off late game

Every melee falls off lategame, because they are melee and have to get in the middle of the enemy team to do serious damage. During the midgame this isn't really a problem, because there isn't enough damage to punish this.

Don't think talon falls off late btw

Lee because most of his damage is single target and he can't get away with building full damage, however his outplay potential keeps him relevant.

Aatrox because he has shitty base stats for a melee auto attacker, lacks burst and is quite easy to burst down and CC.

Aatrox falls off late mid game it seems. Lee Sin needs to fall off since he is so strong early and mid. Vi, doesn't really fall off at all. She is super tanky, decent damage, and CC. Zed and Talon can still delete carries late game no problem at all.

Bonus AD ratios vs total ad ratios

You're saying a bruiser with %hp shred with 3 hard cc, armor shred, AS steroids and a passive shield falls off late game. Yes, better buff Vi.

aatrox doesnt really fall off because his early is not that good, his cds are way too high to make viable, Q is easily interrupted, and passive basicaly brings you up with nothing to fight back with so you just die, it needs to bring you back with full pool and cds up (except ulti )

zed and talon are at their strongest points late game so no idea where you are getting this "falling off" idea from

lee sin does not actualy fall off, its simply players being bad ( he can easily 1 shot squishies and get out with 3-4 tank items ) Vi needs to build a bit off damage to still be able to 100-0 squishies late game and most either do that and get shreaded in bad engagements or build tanky and fall of because their team sucks and she cant drag them to victory anymore