Yes, there are people out there using slack as the alternatives aren't that better. To use Microsoft teams the entire company needs to be on office365 and teams isn't that integrated with every service yet. Google chat is supposed to be cool but it has the same issue.

Most of my clients use slack in their companies with teams from 20-25 to around 600.

My very large company uses slack and Microsoft teams. Mostly engineers use slack, managers mostly use teams.

It's probably because of how much nicer slack is to use.