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.
Surely it’s more efficient (for both parties) to type and be able to read the whole thing and then respond meaningfully?
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.
But I find THAT attitude to be quite rude. You are prioritizing your preferences when it's me that you're reaching out to for help. Nobody's saying you have to write a complete and detailed problem description in your first message, but give me something to know what i'm getting into.
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.