The simplest answer is the material conditions of having one's direct material interests conflict with other people's rights and survival. It's a sort of cognitive dissonance resolution thing, that when someone is benefitting from hurting someone else they'll start trying to rationalize it: it's their "right" to take this for themselves, it's the "natural order" of the world that they should receive and others must suffer for it, the ones who are suffering are wicked and deserve to be hurt, and so on. Give that sort of mindset generations to fester and stew and you get it formalized into all sorts of violent reactionary ideologies. This is true whether one's talking about bourgeoisie, landlords, privileged ethnic groups, men in a patriarchal society, or settlers.

The specific arrangement of settler colonialism is also effectively a trap: the settlers' entire way of life and continued comfort depends on the brutal order of extraction and theft continuing, because their homes are on stolen land (and sometimes are literally homes that were directly stolen as-is), because their jobs are on stolen land, because all their wealth is tied up in systems that rely on the settler state. This means that an end to the settler project and a redress of its crimes means they lose everything and are destitute in a reactionary system that unpersons anyone who lacks land and wealth, that they would become refugees and find themselves at the mercy of other predatory capitalists eager to exploit and destroy them for profit. Considering that engaging in a genocidal settler colonialist project in the first place means starting from a position of already having a genocidal supremacist ideology, you can see how it only gets worse with time as the material conditions make it entrenched and more costly for its perpetrators to stop.

Edit: fuck this is reminding me of a "textbook" on military strategy I read when I was in highschool and was the sort of insufferable nerd who'd read stuff like that, which apart from having galaxy brain takes like "partisan resistance movements are a bad strategy because 'they are not militarily useful' and further 'corrode social morality' leading to restless populations later" also literally talked about and praised settler colonialism as a method of conquest for rather similar reasons to those I'm condemning it with here, that the settlers necessarily must be in conflict with the natives and so would "be more loyal" to the core that they both rely on and have cultural ties to than a subjugated native population would be. I looked around to see if I still had it so I could look at its brainworms through the lens of marxist analysis I have now, but its not on my bookshelves so I think it must be one of the ones that got ruined or lost while I was moving. I'm just struck by the memory of how it was basically taking a correct material analysis of some things and then applying the most ass backwards moral valuations to it to the point that now I'm wondering if it wasn't written by some trot turned neocon or something.