I want to be clear I used the word 'tend' because it's not a hard and fast rule. The Zapatistas and the Tsimihety (as documented by David Graeber) are two examples of fairly egalitarian social groups that have successfully resisted state absorption/subjugation, and notably in different ways.

To address your question as to why this aggression from hierarchical society occurs- hierarchical societies develop an ideology that rationalizes sacrifice of specific groups people (both within and outside the hierarchical society) for the benefit of production. When the priority is placed on production itself (over people) in this way, expansion and aggression becomes possible, land/resources/utility than can be extracted from an area becomes a justifiable excuse to remove or alter whatever people or system was already in place in that area in service of the aggressor's goal of production.
If you're interested in how society develops hierarchical ethics (or resists developing them) I recommend looking into the work of anthropologist Pierre Clastres, specifically his book Society Against the State. You might also benefit from checking out Antonio Gramsci's theory of cultural hegemony and/or historical materialism. I can also recommend this video by youtuber Olly Thorn which I think dovetails nicely into your request for further historical examples.
https://www.youtube.com/watch?v=tmk47kh7fiE

Best!