That's because we're communicating synchronously in person. If you say something when I'm not listening to you, I will probably start listening midway through your statement, and miss potentially vital info. In a slack message, I can just read it again.
IDK about you but I get chats from 30+ different people and I usually miss at least one person's message a day as it falls off the "front page" so to speak
I don't see how "hello?" helps with that. If anything, it makes things worse if everybody does that, because now half of those chats from 30+ different people are that, drowning out the useful messages.
By the way, I also hate the "hello"-only message. I am, however, guilty of writing "Hey. Do you have a second to chat" - typically in cases where either through chat or video conference I want to go through something that is more involved, and I also want some confirmation of understanding and acknowledgement.
If the notification bubble just says "hello" it's on the bottom of the stack of my priorities. If it's "hey, this alert came up..." then it's actually going to flag my attention.
If you want my attention give me a reason to give it.
If it’s urgent enough that the actual message isn’t enough, “Hello” isn’t going to cut it either.