Why do people without autism exclude people?

I would assume it's been bred into the majority of the species.

Imagine the in out group dynamics in hunter gatherer times. The out group at that point would have been absolutely competitor's for the same resources, plus, likely potentially very violent. Survival, on the gene level likely meant "be hostile and wary of outsiders". Its also true, than safety lies within the group, so to be ostracised was literally death. No surprise to see these defining nt traits.

Be an outsider at your peril. (I'm feeling particularly outsiderish today unfortunately). 

It wouldn't be so bad if it were done to your face, but it's done silently, passive aggressively, and you only get a vague sense something is wrong and you are excluded, when it's too late. /vent.

Because of ASD I have disordered perception.  I perceive as I am not as It is. 

They say don't they that humans are genetically 90% chimpanzee. I think.its probably an ape thing. Creating a pecking order of who is dominant who isn't, then who is, one of us, and who isn't. When two people get together they can feel more solidarity by badmouthing someone else.

But maybe I'm getting cynical. 

Not many Neurotypical people like people like others who are outside the “norm” like autistic people. That’s why I prefer to talk to people with neurological conditions like me.

I don't think it's so much that they are looking to leave someone out as find people with things in common with them. Unfortunately people with autism are often quite unique so regularly don't have things in common with others.

It isn't just those without autism that leave people out though. Autistic people can do this too, I'd assume for similar reasons.

People aren't going to like everyone and friendship groups can be good.

Having said all this sometimes people just need to take more time to get to know others before making judgement and think about how excluding someone from something can make them feel.