> And culture is, by and large, random, arbitrary, and self-reinforcing. The best definition of "culture" I've ever found is "how we do things 'round here". It's valid in both the large and in the small. Of course, why and how we converge on those norms is mysterious, and the anthropologists, the psychologists, and etc. can have a go at explaining those parts. I can't.
Cultural baggage, for the lack of a better word, drives how we tend to approach reality (holistically or by dividing and classifying things, monistically or dualistically, materialistically or idealistically, and so on), and reality includes the very thing under discussion (consciousness, culture).
Shared cultural baggage is perhaps the thing that makes us believe another being is conscious (i.e., shares similar aspects of self-awareness). Shared culture manifests itself in an infinity of fine details of one’s behaviour; looking like a human but not behaving like a human can be a great horror movie trope, depending on how carefully shared culture is violated[0].
This carries over to animals, to a degree. A dog is social to an extent that many would consider it conscious. An octopus is legally recognised as sentient in some countries—thanks to it behaving in a way that is vaguely reminiscent of ourselves. Same reason we call ravens smart.
Most humans anywhere on the planet, though, share enough cultural baggage that we do not question whether others have what we consider consciousness; though I think some people are more sensitive to how much shared cultural baggage another human possesses, the small lack of which could lead to fear, cautiousness, and/or a feeling that they are in some ways subhuman (closer than a dog, but not as human as their peers in local community) relative to them, which eventually contributes to exclusion, racism, and so on (well demonstrated in both Japan and parts of the US).
[0] Arguably, “behaving sufficiently like a human while being not human at all”, which we have plenty of examples of now in the last year or two, is another such trope.