But the setting defines a lot of the context for the 'welcoming' eye contact.

If you're walking down the sidewalk of a neighborhood street in the daylight and see an individual walking in the other direction, both people are more likely to be friendly to each other than if that encounter happened at night.

But even if the other person doesn't make eye contact that doesn't necessarily imply a lack of respect or regard. For instance, if I'm walking home from the bus stop at night and I walk in the opposite direction of a woman, I won't make eye contact with her because I wouldn't want her to think I'm leering or have an alterior motive behind the making eye contact. My intent is for her to feel safe, even though we won't have any interaction.

But I think you're wanting a reason why people don't feel comfortable making pleasant eye contact with random people. I don't have an answer for that question, tho.