So i think this is a very interesting question that can be approached from several angles, because while one could just talk explicitly about the examples you just mentioned, the question really strikes at the historical and sociological conception of warfare as a whole, and it's indeed very difficult to understand premodern warfare through our modern lenses, since our conceptions of violence are radically different in this modern age. i'd dare say, war, war actually does change.

As prussian soldier and military theorist Clausewitz famously stated: "War is the continuation of politics by other means." which might resonate with us today as war is an exclusive prerrogative of the state, and we have a lot of funky innuendos to describe non state military actors, like "terrorist" "paramilitary group" "rebel" and whatnot, and war is actually severely regulated, as it seems military actors alwayss have deep, complicated reasons to go to war, based on a host of justifications ranging from nationalism, religion, political ideology, economic and strategic interests etc. But in reality Clausewitz statement is a severely culturally loaded one, that might aptly describe the military realities of the world he knew, but doesn't seem to be of much help when dealing with different places and different times.

Case in point, the examples you point out, in which warfare is endemic and not seemingly waged for the purposes of advancing a large geopolitical strategy or defending the interests and sovereignity of a nation, like state rationale would dictate. Of course, the modern state didn't exist, although centralized political entities certainly did, but war was often a very descentralized affair, in which every step of the way, from recruitment, to logistics, to command and control, to army composition, was not really something that a premodern state often did itself, but cobbled together from a host of different mechanisms based on political and personal loyalties in which all the different actors stood to gain something for themselves instead of merely functioning as a tool to achieve the state's bidding.

And who are these autonomous actors? Their nature is extremely varied according to time and place, but there's a few patterns we can note. On first instance we can note a particular socio military caste that we can merely call a warrior culture, which refers to a particular social group whose literal job is to do war, they are expected to fight, trained since youth, and engage in conflicts of all types at all times merely because that's what they're meant to do. Military castes are very common in history, such as the European knights, the Japanese samurai, the Hindu kshatriya, the turko-mongol horse warrior, and many many more. These groups engaged in war so much as there were many benefits to be had, the riches of plunder when conquering enemy territory, the granting of lands and political power in return for their service, and the fame and glory associated with succesful military exploits which were the very basis of the culture they existed in. The caveat is that these troops don't necessarily have an allegiance to a state, but rather to their own lineage (which itself can be culturally defined in different ways) as well as personal alliances that are ever changing. So in several premodern societies you literally had a dedicated class that would go out of its way to wage war for profit, for power and for glory, and not necessarily in a a centralized way, as allies in a former campaign could later become bitter enemies over the division of the spoils of war. Do bear in mind that not all military operations were epic wars of conquest, not at all, many times these groups were content with merely raiding and plundering border territories and carrying off with the loot before effective resistance could be mounted. In this you see examples as varied as the chevauchée of medieval Europe, viking raids from beyond the sea as well as nomadic raids from beyond the central asian steppe.

Another system is the militia system, in which soldiers are expected to provide military service in times of need, and militias can vary from part time soldiers called up to beef up the main army, to a highly patriotic force heavily invested in the defense of the community and forming a class in itself. The greek and early roman armies, as well as the later swiss pike infantry, functioned in this manner, and just like above, for them war was a means of social advancement as well as enrichment.

Then there's of course mercenary troops, that are literally there just for the profit. In traditional western historiography mercenaries are often seen as unreliable, this is not necessarily true, after all you wouldn't be hired if you coudn't be relied on in the first place, but mercenaries were often hard to please, as states often hired them since they only had to be payed during camapign season and even then they could still have trouble with payments, in which case the mercenaries could mutiny, so sometimes instead they took plunder as payment, and sometimes even took territory for themselves, as the crusaders of the 4th crusade did in Constantinople or the ancient Mamertines who took over the city of Messania, which would eventually trigger the first punic war. Other more irregular troops like the Cossaks fought mostly for their own benefit, shying away from any encounter in which they could face many losses and preffering to loot and raid, and promptly abandoning their masters if things got out of hand

So all of these examples basically show one simple thing: that there were often personal and private interests in the conduct of premodern war, it was a very profitable enterprise for those brave enough to jump into such an adventurous lifestyle, as it could bring not only wealth, but even political power as well as social standing, since the culture of the day very much saw warfare as a noble endeavor worthy of praise, and the hardships of war created a sense of community and belonging among those who fought together through thick and thin, justifying and perpetuating a martial culture among them.

Essentially war was extremely unregulated and descentralized compared to what we see today, and those who waged it were not necessarily affiliated with large impersonal state bureaucracies and their complicated strategic goals, some were there for the plunder, some for their social standing, and for some it was quite literally part of their cultural tradition to go to war at every chance they got.

This main source for this answer is John Keegan's "A history of warfare" which is a great book that challenges traditional perspectives for understanding warfare, seeing it as fundamentally a cultural phenomenom that takes many shapes accross history