Different people have different reasons but I notice that many people that do a naked "hi" / "hey" are trying to see if the receiver is present for an immediate reply.
In other words, the sender deliberately wants a synchronous instead of asynchronous convo.
They don't want to type out a bunch of words for a question if the reply isn't going to happen for a few hours.
EDIT to reply: "Then send it in an email."
To clarify, some people don't want an async communication so switching to email doesn't solve their problem at all.
The issue is that some personality styles do not like sending questions into an indeterminant void. Thus, a naked "hi" for them is shorthand for "r u there to talk to me RIGHT NOW?"
I sympathize with their need for immediacy but I made a previous comment on why it drives me crazy: https://news.ycombinator.com/item?id=17255452
Yep, some people are really confused as to why many async-preference people like me do not want to answer the phone.