For most Security Leak issues, it comes down to a simpler problem.
Most people have crappy computer skills.
You can have a perfect system, but it takes one guy from sales or marketing to take the data, dump it as an excel of csv file and just email it or drop it in a public space because he just doesn't want to be bothered by dealing with IT
XKCD kinda shows this problem. We still don't have a good way to transfer files with people on different network. We have the technology but no clear standard.