The privacy statement of Slack is horrible. I also wonder why so many companies trust often highly sensitive conversations to what is essentially an IRC forum that could be taken over by an evil company any minute.
Convenience unfortunately seems to be higher valued than privacy by many. This held true for a web agency I worked for. Getting things done, even with proprietary tools, stood above all. The privacy policy was read later, or not at all.
[Many|most] companies big enough to worry about company secrets and PII being logged will get a contract with Slack set up before putting anything on there.