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.
It makes more sense in the historical context: 4chan was incoherent libertine-slanted counter-culture shit for about the first decade of its existence. Like when I was in highschool all the queer theater kids and general outcasts were avid 4chan users, and while it was still awful then it's not like the mainstream norms of the time weren't as bad or worse. That's sort of what western counter-culture has been like, just a bunch of incoherent self-serving libertine shit that hated the establishment because the theocratic mainstream was getting in the way of their treats, and in general counter-culture is where new culture comes from even if it remains on the fringes itself.
The problem is incoherent self-serving libertinism has no real revolutionary or even progressive potential: where it pushes for liberation it does so because it wants to be free itself, and it is just as capable of being reactionary and chauvinist as it is anti-establishment when it inevitably comes to perceive the left as a bigger threat to its treats than the establishment. And that's what happened with 4chan and the broader counter cultural movement it was part of: the far-right propaganda funding temporarily shifted from theocracy to astroturfed fascism and it successfully convinced a whole bunch of self-serving chauvinist treatlads that the right was no longer coming for their treats but that women and minorities were, causing a schism where everyone in the sort of vaguely-left-by-american-standards counter culture either reformed and moved left or became a babytalking anime frog nazi, with only a few weirdo grifters like v**sh trying to keep the pre-gamergate chauvinist libertine sucdem counter culture alive.
But also there's the fact that people on 4chan were extremely terminally online and so just spread things by also posting aggressively everywhere else. It's like how r/chapotraphouse had a massively outsized presence on
because we were all just terminally online and posted everywhere.