I can’t explain to you why you find it annoying. I honestly don’t. What’s annoying about asking people if they are around? Or asking a question in a channel?
Maybe in your org people are better communicators. In mine, Slack gets in the way of deep work. Then you turn off notifications and eventually your manager asks you why you are not on Slack.
So you end up reading memes, news, answering same question 100th time, and you realize you are just glorified assembly worker. You are not solving real problems, just making another distraction for your primate family. You are a monkey!
I mean, apart from anything else it's literally exactly the same issues you were highlighting with email: unnecessary verbiage to warm the discussion up and copying in loads of people indirectly involved, which may or may not be related to internal politics. And the fact the discussion which actually resolves the issue often won't take place in the channel.
With Slack you've got the added irritation of an expectation of real time responses, especially when someone's @mentioning the boss or everyone else in the division a minute after you haven't responded...