Not so. Nothing stops Linux distros defaulting to mail and file systems that encrypt everything by default, but the reality is that most people can't be bothred. I certainly can't, and I don't feel like putting myself out to encrypt everything in order to make it popular. As has been pointed out, the metadata of who you email and phone, while not probative in the same fashion as the contents of calls and emails, are nevertheless a significant source of data, and encryption won't alter that without major changes to the architecture of mail.