Amother organism or person needs to be predictable to be trusted.someone or something acting in an unpredictable manner means it may suddenly decide you're a threat. We evolved in a world before science and medicine, where any injury could mean death. Even the most unstoppable animals, bears, elephants, moose, will bluff charge a threat rather than immediately attack, because fighting risks injury, regardless of how unbalanced the fight is. I can't win a fight against any of those animals, but I can bite it while it's killing me. A full thickness bite wound is all but guaranteed to cause an infection, which may kill or disable.

Humans are also social creatures, and we run on cultural norms that make it easier to trust that the person next to you in a restaurant won't suddenly stab you, even though he is holding a knife.

A major cultural difference can make others seem dangerous in a primal way. We know through interaction that other cultures are not more dangerous, but that primal unease of being surrounded by people from a different tribe is still in there somewhere.

In my opinion, this is why racism is so hard to root out. A lot of it is taught by others, but it's not a negligible amount tied to fear of anything different.