I think it is more of a if you are not there right now, and won't be able to respond, I am not going to write it all to wait for an answer later. I think most people want to make sure someone is there to respond before committing to a conversation.
BAD: Hey, you there?
GOOD: Hey, you there? I'm trying to do X but I'm running into some issues and I wanted to get your advice.
Once I've responded and you know you have my attention, then you commit to filling me in on the gory details.
E.g. If you’ve just say “hi”, two hours later I get to my DMs and say “hey what’s up?” and you end up not following up with the “actual” message straight away, let’s say another hour later, this all took way longer than necessary.
The no-hello approach just makes sense when dealing with asynchronous messaging platforms such as Slack. IMO, not following the no-hello approach is bad etiquette and there’s a ton of people out there who still don’t really get that.