But Outlook lets you mark an email as "do not forward". That makes it safe, right?

Documents in many places complicates things. For example, if you have to go through discovery, all of a sudden you have to make sure you find things on every single service you've stored them. That's (part of) legal's job, and the number of different services you're using is a multiplier on a lot of things.

What gp described just seems inefficient in general.

It's more that the greater number of channels the more difficult it is to harmonize corporate policies across all those channels. For example, communicating via email entails data retention for a period of time as a matter of law these days (SOX, etc.) What happens when people start using a shared dropbox instance to send, say, todo lists, short messages, power point slides, tech specs, etc. to each other? What's the intersection with the types of communication which are sufficiently "email like" to trigger a concern in terms of data retention? And then that brings up a whole host of technical issues and so forth.