There are two categories of marksmen. Ones that rely on their abilities or their kit in general, and the others that rely on their autoattacks and strong scaling on them.
The first type includes Graves, Quinn, Gnar, Corki, Kennen etc. They all utilize abilities to be effective rather than just straight autos and are more of the burst type than the constant high DPS. Quinn utilizes the empowered AA on champions to deal a big burst of damage with her E AA Q AA combo, but then, she lacks any serious follow up.
The second type however relies on autoattacks mostly. Even Lucian, who uses abilities, is useful because of his passive double-shot which is an AA augmentation. These champions have extreme scaling because their AAs get a lot of attack speed or attack damage, which is destructive late game because of crit itemization, while they generally have decent or high AA range (Jinx, Caitlyn, Kogmaw, Varus, Ashe).
Graves can be placed in both categories, based on the way he is played. He can be played as an assassin caster with thunderlords, which is more used in the jungle for maximum early game powerspike with Lethality, or can be played as a traditional ADC with Warlords and crit, which is viable top lane.
Generally, champions that rely on their abilities to do damage are stronger early game and the AA ones are stronger late game, because abilities have base damage values that add burst, which is potent when the enemy only has 1000 max HP in the early game, but falls off later when a tank for example has 4k HP. At the same time, using AAs to take down someone with 1k HP can be hard early, because you have to invest like 10 AAs and that's a lot of time.Later on though, the AAs can deal 1k damage each for example, which means that shredding down anyone isn't as hard.
That's why the traditional AA marksmen can't fare well in solo lane environment. They don't have the tools to survive a lane vs an assassin or a mage or even a tank.