Constant War of Small Medieval States requires mercenaries and a mercenary outlook.
Time Periods featuring small states with shifting allegiances especially prior to consolidation of a land mass by one power, offer endless opportunities to be at war. Even though an elite class of warriors may exist they might easily have shifting loyalties several times in their lifetime forming new units and affiliations for different battles.
It is necessary in a medieval world, that a state sends its warriors to show support for other states, to push their family further up the hierarchy of noble clans and remain in favour. Keep your enemies close, as they say.
Even though Samurai are famous for their loyalty, in the 1500s and early 1600s the daimyos would often commit their troops to other daimyos, and if they lost a battle or were betrayed by a general, all the samurai committed to one side would then go to another. The fighting was continuous for almost a hundred years.
Consider all these permutations of Japan: Nobunaga,killed his own brother to take control of his clan, fought and destroyed Shingen, was betrayed and defeated by Hideyoshi, who invents a lineage to take control of the country, unifying most of Japan, and then his son Hideyori is defeated by Leyasu, who finally completes the task of uniting all of Japan and assembles an elder council of other daimyos to ensure that his dynasty will stay in place.
Even when a ruling dynasty occurs, they will levy troops from smaller, poorer houses . On the other hand, as long as there is no stable leadership, there will constant statecraft, and a dynamic leader will always adapt and have a mercenary outlook.