I mean...come on Demacia.....Everything isn't all swords and shields and straight up confrontation.
Of course not! There's also scouting, crossbows, cavalrymen, lancing, fencing, there's a bunch of ways Demacians go about fighting. They're just not about being sneaky or subversive about it. The closest champ to embody that kind of idea is Vayne but hers is a path more alligned with athleticism; she probably doesn't actually go invisible or anything, unlike say Rengar who has his camouflage thing as an active part of his biology seemingly.
I mean, there are no Freljordian assassins, no Shuriman assassins, no Bilgewater assassins (Fizz doesn't count since he ain't a hired killer), and no Piltie assassins, so y'know, it's not like Demacia's alone in that. Noxian culture's probably just more conducive to that line of work considering their approach to the ideals of strength and power.