It happened naturally in CK2, at least it did for large empires.
CK2 had a functional levy system, so raising your entire army was very expensive, pissed off every single vassal under you, and gathering those armies would take very long, so people wouldn't raise every levy from everywhere just to fight a war at a certain flank, it was more reasonable to simply raise some levies from neighboring lands and use them instead.
Then let vassals forget their negative opinions for a while, cooling off as you'd raise another part of your vassal's levies, on another side of your kingdom, to fight a different wat somewhere else while not raising the first side's lievies.
CK3 butchered every single area of the levy system so every nuance is gone, there is no reason not to simply raise all, all the time, and move your flag around, troops teleport at light speed, they don't suffer attrition on the way, they can't be intercepted, they don't piss off your vassals.... And they don't even work past the first decades of gameplay.