They were too small. But they had their own social orders of equivalent importance, and breaking those would break them apart. There's a reason religion and tradition played bigger role in a distant past, and going against them was severely punished. It's not just out of spite or "us vs. them"; people take threats to stability of their group personally. It's definitely in part a survival mechanism.

> The evolution of the modern state is a result of inter society competition for who can apply the most massed violence against a competing state.

Yes. More specifically, it's the result of growth. It's the same thing as small tribes fighting each other over some small areas of land, except scaled up. Bigger groups have a competitive advantage over smaller groups, but there's a limit to the size of a group beyond which it ends up splitting apart; increasing that limit requires stacking more layers of hierarchy and associated social technologies. "Rule of law" and the legal system in general is one of such technologies, and it looks like it does today, at scales of groups we have today.

A group of dozens can just work on instinct alone. A group of hundreds requires some rules and specialization and designated authority. Scale that 100x, and you need another level of leadership hierarchy just to keep sub-group leaders coordinated and aligned. Scale that 100x further, and you kind of have to get something looking like a modern nation state, as anything else would either break apart or be defeated by another group that is more like a modern nation state.

See also: Dunbar's number.

> We've seen what happens when empires fall apart (Rome for example) and things don't revert to "prehistorical lifestyle of small tribes slaughtering each other for what little scraps the land has to give".

Europe would disagree.