Because they don't seem to make it easy. It doesn't seem as a individual user I have any say in how my data is used, I have to contact the Workspace Owner. When I do I'll be asking them to look at alternative platforms instead.

"Contact us to opt out. If you want to exclude your Customer Data from Slack global models, you can opt out. To opt out, please have your Org or Workspace Owners or Primary Owner contact our Customer Experience team at feedback@slack.com with your Workspace/Org URL and the subject line “Slack Global model opt-out request.” We will process your request and respond once the opt out has been completed."