The Walt Disney Company is reportedly ditching Slack after a July data breach exposed over 1TB of confidential messages and files posted to the company's internal communication channels.
NIH isn't healthy either to be honest. Plus when some service fucks up, you may have legal recourse. If it's your own employees, you can fire them but won't get much money out of them.
Self-hosting isn't some silver bullet. Poorly self-hosted stuff is way less secure than large corporate cloud-based services. There are very few organizations that can successfully craft a secure IT environment for themselves.
Data can be leaked from self hosted services as well especially if you don't have people with he skillset and management that backs up the security team.