Other than stuff like Pergamon being given to Rome in a will, kingdoms being united through marriages and stuff like that, you have very complex relationships that involve military and diplomatic conquests. Roman Italy itself was not integrated until the wars were long forgotten. It was basically a network of city states owning colonies and having Vassal city states themselves (not unlike a Feudal system like in CK 2, but based on cities and not nobiliary titles). The move towards more integrated states was more social than military. Wars were a means to achieve initial overlordship, but not always direct integration.
CK2 model of expansion is not that far from what an early history looks like. You certainly have more Republic and Empires here, but the basic style of societies (tribes, nomads, kingdoms, city states…) are here, just without the religion and messier/more distinct.
Here it’s just too easy and convenient to assimilate directly through conquest, because there is exactly one single diplomatic stance that allows any progression towards integration. When you can select it as a war result you can just take the province anyway, so it is not useful, and it’smalmost impossible to get there with the diplomatic tools at your disposal.
As Carthage I tried to play a diplomatic game making Vassal Tribal Knigdoms (which is just doable), only to discover that:
- It’s a dead end that does not allow further integration.
- there’s a hard limit on the number of Vassals you can have. The diplomatic option disappears after you have enough.
The way to progress? Fabricate a claim, give them freedom, declare war, annihilate them and conquer them into your territory.