Because its hard to define the parts that are really sensitive. At our work people must classify every document but a lot of people choose public for everything because it doesn't enforce any restrictions. So they can just dump it in a folder and share it with the whole company. This is not what we want them to do obviously but people are lazy, don't like to create access lists. But anyway it means we can't rely on the classification. And indicator detection like credit card and social security numbers is far from perfect. A lot of sensitive info will just be text, like about new products being developed. 3D models, code, strategy emails.

Also, if people start rooting around in everything they can take things out of context. If I send a message to my boss that I think that something we're doing is stupid, if that were public it could make some waves even though internally it's inconsequential because I'm a nobody. Also, many documents might have one or two bits that hint to really important information and having them can help finding those

As you probably know, there's tons of information in a multinational and the hardest part is finding the right stuff. This is one of the main tasks I use Copilot for. Also because outlook and SharePoint search are really terrible though. If those actually worked I wouldn't need copilot so much.