Because a tribe is a real organism, not just as a metaphor, and it is bound together by ideology, and less well bound tribes did not survive. Ideology is a kind of muscle at a higher level of the fractal we inhabit.
Article shows how hard it is for academics to do genuinely neutral research that might yield insights accepted by all. She appears to think of herself as an unbiased researcher but her whole perspective is deeply biased e.g. when discussing the larger amygdala in people on the right, she takes it for granted that the left wing brain is normal and the right wing brain is abnormal, but this stance seems to be based on nothing. A right wing person can just as easily argue that their brain is normal and for people on the left it's atrophied, and they've done exactly that when this difference has been brought up in the past.<p>She then describes people on the right as "chilling" and "numb to injustice", and those who convert to religion as psychologically "rigid". This is based on observing that people on the right don't get emotional when shown a video that's explicitly designed to trigger people on the left.<p>Meanwhile the center-left (presumably where she thinks she is) is described as "creative", "flexible", "moderate" and "least likely to latch onto group identities".<p>All this stuff sails dangerously close to being circular reasoning. Ideological people are more mentally rigid - that can be phrased another way, as moderates are easily pushed around because they don't think hard about what they believe. Flexibility sounds less virtuous when phrased like that, but it's not a more or less valid presentation than what she's doing. All this is far from being scientific.
I've had a book in my life for a little while now, called "I Am That". It's about exploring the space before the "ideology" takes hold. Really enjoying spending time in that space during these times.