I think it comes from a deep threat response. People generally I'd say are quite insecure, they look for a majority to agree with them rather than walk their own path, and I feel this is done by a continuous need for validation. Perhaps it's a societal thing, it releases dopamine when we are praised so we constantly search for it. You're more likely to find it in a group that shares your interests/views etc. "we're from this country" give us a solidarity to a stranger we'd otherwise not have concerned ourselves with but we seek their validation of our behaviour or life choices. I think it's a taught behaviour for people to be so afraid of failing or being wrong they'll look to whoever will back up their narrative, just to get that dopamine. Being wrong or being outside the consensus can bring on a lot of anxiety for people. I think there's some avoidance of that sometimes. I think tribalism comes from an instinctual need to belong somewhere, you're safe when validated because it means you're not alone, and being alone is dangerous in the wild. I think humans have developed technology faster than we've evolved psychology to keep up with that technology. We don't need "people on our side" to be safe anymore, yet we seek it because that's instinctually driven.

As an example of technology moving faster than human evolution, take a look at obesity rates as an example. By storing more energy as fat than your body needs it displays that our bodies still prioritise energy conservation. With this in mind, I'd say that the ability to get food delivered at the tap of a button means you've expended less energy than you've gained. You're in an energy surplus but your body is reserving that fuel with the expectation that you'll need to fight or flight or even hunt/forage.

Went off on a bit of a tangent but to wrap it back up I think our psyche is still rooted in an instinctual need to be safe and to us numbers = safety so we look to any way we can to increase those numbers, and we also get a socially taught dopamine rush from a validation when someone agrees with us.