In the phrase hi there, there doesn't have any meaning. It's just an extra word that can be added in casual conversation. So it's not that there means here in that situation. If you did use here instead to say hi here it would sound strange.

Its implication is that the person was not "here" with you; they are entering your presence (here) from somewhere else (there). You are welcoming them into your presence (here) by acknowledging them from elsewhere (hi, there). Interestingly enough, I looked up 'there' in a dictionary and found that in this context, 'there' has its own definition: there T͟Her/ adverb adverb: there; existential there: there 1. in, at, or to that place or position. "we went on to Paris and stayed there eleven days" 2. used in attracting someone's attention or calling attention to someone or something. "hello there!"