Because there's different sub categories of marksmen. There's supportive marksmen that provide CC like Ashe and Varus, early game dive your face marksmen like Lucian and Kalista, and late game hypercarry marksmen like Sivir, Twitch. Marksmen have sub identities within themselves already.

More importantly though I also don't understand this ridiculous notion that all the roles need to be treated in the same way? That's like if mid laners suddenly got no gold and people were like "well it's fair because supports used to have to deal with that" it's just not how the game has grown to be understood and the formula they already had was proven to be a winning fun formula already.