ADCs

Why can I dominate as Kayn mid game then I just get completely fucked by a marksman who right clicks me 3 times even I after I combo perfectly? 1 right click resores al lthe damage I did with my Q and by the time I end my ult they're full hp. Why don't marksmen require any skill other than positioning which is more common sense than anything? All champions require positioning if that's the case but marksmen are just braindead right clickers late game and it's not fun to play against.