Why not? There's inherently more counterplay and more skill involved in champions that both cast skills and autoattack, and if they can manage to correctly build and play well then they should be more powerful then a basic autoattack ADC when their skills are off cooldown.
Skills themselves don't offer more counterplay by default. "Don't get hit" is a little different when you're facing a 1100 range skill like Mystic Shot compared to a 550 range auto-attack. Also, auto-attack steroids like Final Hour can easily be countered by not being next to a champion, which can be accomplished through mobility, positioning, and/or crowd-control.
Generally speaking - skills that front-load all the damage you could have done with auto-attacks in a single instance as well. Rather than needing to maintain solid positioning where you're close enough to deal constant damage but in a safe position for ~6 seconds, with burst you can just unload everything in an otherwise perilous location and then dodge out a second later. Even if you get wombo-comboed, unloading a bunch of front-loaded damage will often allow you to do your job in team-fights.
With nothing but auto-attacks alone, you get bursted immediately and your team's at a big dissadvantage. That's mostly the TLDR on why marksman are often focused before mages - mages AoE actually makes them stronger in team-fights, but they only need 1 second to pull their weight.
Doing nothing but auto-attacking is actually pretty easy to play around. Focus fire, burst, CC - pretty much any aggressive action taken against an auto-attacker is the right decision. It's one of the reasons you basically never see AD Sion.
Also - concerning seeing champions, caster marksman have been a dominant force for every season but the current one. In season 1, Tri-Force Corki was considered the best marksman. In season 2 it was Tri-Force Ezreal, fallowed shortly by Corki and Graves. In season 3 there were a lot of changes, but with a re-vamped Tri-Force right before worlds - Ezreal and Corki were the top picks again. For this season, Lucian, another caster-ADC, was shaping up to be the dominant pick.
Which is fairly awkward concerning "marksman", the most committed auto-attacking roll has been dominated by very caster-heavy champions.
And it makes a certain degree of sense - if you can burst hard enough it's worth using over auto-attacks and then fallow that burst with some solid auto-attacks, champions like Vayne and Kog'Maw that are pretty much limited to auto-attacking alone are already chunked to 50% health before they can respond.
Mid Tier Item: Scythe of Mind Stealing
My main problem with Reaver having a mid tier item is this:
Jayce was the most broken champion in the game when he could stack a tear by stance-dancing. Making mana-dependant champions manaless with a minimal gold investment really throws balance off.
Most of the mana-dependent champions are balanced around a limited ammo count early-on. And generally speaking - mana costs tend to correlate very strongly with how front-loaded skills are. The more mana, the more they tend to be akin to blasting a foe with a rocket launcher over machine-gunning them to death.
Anyways - both Muramana and Reaver require mana costs are a real gate to a champion's power early-on, where they transition into an item that offsets mana sustains rapid diminishment in importance once the lanning phase ends by essentially offering all the mana-sustain for free upon the completed item.
Nobody wanted manamune when it had no upgrade as the mana costing gold set back the damage potential of auto-attackers, which can really feel that hit with their exponential scaling. You kind of need to offer the mana sustain for free at some point for the item to be worthwhile - but offer it to early and big mana costs don't serve as an ammo system at all.