I think, basically, because we didn't learn to.
And, along with that, "keeping eye contact" isn't as simple as that wording makes it sound. Because it's keeping eye contact for the right length of time. Keeping eye contact too long is staring, which most of the time isn't appropriate.
So, it's a matter of learning, on a subconscious, innate, level, that we should make eye contact, and for how long, and the whole pattern of it in a conversation. We can't do it if we haven't learned it.
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not aspie, not NT, somewhere in between
Aspie Quiz: 110 Aspie, 103 Neurotypical.
Used to be more autistic than I am now.