Who knows, I play Materialists half of the time and still see the Chinoor, who are also Materialist fools, very very often.As far as I can tell from the game files, it's supposed to be random. But I wonder if there's some balance feature that's causing it: maybe the game throws in couple of premade Militarist empires if you're playing as Pacifist... So you won't have easy victory with federations.
I should probably add in some premade species of my own, just to make the default ones rarer. Since full randomness isn't an option.
I did once get a Blorg FE, that was funny.
For now, I'm gonna try making a bunch of empires just hitting the random button on every screen in the creator and see if it makes things better next time.