Marksmen is a thing, and it's pretty inane that you think it feasible to just re-word every Champion in the Marksman class as Skirmishers. Especially since Skirmishers are defined here as part of the Slayer Class - "Fragile, agile, damage-focused melee Champions", with the Skirmisher Subclass being defined by high-survivability, few reliable gap-closers, and being intent to shred anything within reach.
So, do the vast majority of Marksmen have few good gap-closers? Sort of - barring Lucian's Relentless Pursuit, Graves' Quickdraw, Vayne's Tumble, and other such instances of dashes, the majority of Marksmen don't have great access to dive on priority targets.
Do they have good survivability tools? Largely: No.
Are they melee? No. Not a single one of them.
Also, the definition of a marksman in the real world is "a person skilled in shooting, especially with a pistol or rifle." - and wouldn't you know? All Marksmen in League of Legends have some form of ranged weapon, whether it's a bow, a gun, or some abstract launching mechanism like what Urgot has.
Riot has trouble coming up with Marksman Subclasses because a wide variety of Marksmen touch on different subclasses. There's Artillery Marksmen like Kog'Maw or Varus, Snipers like Jhin, Hyper-Marksmen like Kog'Maw and Jinx, Siege Specialists like Caitlyn or Tristana, pseudo-tanks like Urgot and Graves, there's just so many flavors of Marksman that it's really hard to nail a specific few subclasses that define small groups of them well.