Most data traveling in the clear has little value. What value it has may be momentary. A week from now, it is worthless.
Heck, most encrypted data has little value. The fact of the matter is most data is worthless junk.
I was the backup administrator for a Fortune 500 company's branch office of 1,500 users. I have a pretty good idea of what data existed because I was responsible for keeping it safe. Of the terabytes upon terabytes of data sitting in the archive, I could have put the worth-encrypting sensitive company information on a USB thumb-drive. There was really that little of it floating around.
So, the reason most data isn't encrypted is that there really is no reason for its encryption.
Cheers,
Matt