I will never understand people who write "Hello" or "Hi Joe" in Slack and then just wait. It has to be some form of mental illness.
Also, giving context is good, regardless of medium.
If I know the "Hi Joe" is hanging there, part of my focus is holding onto it until I clear it.
I mean, I glace at them at some point, but I don't put much thought into them.
At least I know slack/teams messages are meant for me. If it's important, don't send me an email about it, ping me on one of those.
They correctly assume you don’t need a public reminder for your responsibilities so they opt for a subtle nudge.