There's nothing wrong with social chat on Slack. It just needs to be either in a thread or, better yet, in a dedicated social channel.
Saying people shouldn't have social chat on Slack is like people shouldn't have social chat in the office kitchen because it's part of the same office complex.
And if they did that, I’d have nothing to complain about. That’s never been my experience though with Slack at work.
That’s unfortunate but it’s not a universal trend.
The problem here isn’t Slack, it’s poor Slack etiquette. However you can change etiquette at a company level.