In my experience it’s very common for big companies to not host. Think Fortune 500 type companies. Most are legally happy with their MSA and reasonably confident in security standards.

> In my personal experience, it's very common for big companies to host email, messengers, conferencing software on their own servers.

Mind sharing a clarification on your understanding of "common" and "big"?

Yes, then they use Outlook for example. Have you checked the ToS of the new Outlook version for commoners? They flat out state that they can use all of your emails for whatever they want.

Also, companies host for example an Exchange server on prem; and guess, what it connects to? Why you can usually access account at outlook.com?

I don't think Slack or Notion have on-prem/self-hosted options.

It goes in direct conflict with your idea. I am sure you know some people within your circle that say they cannot leak data but the fact remains. Over 85% of Fortune 500 companies use some combo of OneDrive or Sharepoint. The companies have already gotten familiar with the risks and legally are comfortable with the MSAs. So I am not sure what legs you are standing on.

Absolutely there are specific companies or industries where they think the risk is too great but for many, outsourcing the process is either the same or less risk then doing it all inhouse.