Because calling them metadata provides the government with an argument that minimizes their importance, thereby allowing them to be exploited for nefarious purposes in the same way that calling torture "enhanced interrogation techniques" allows it to be exploited for nefarious purposes. And so we get stuck arguing over the definitions of words instead of the actual actions being performed under those definitions.

At some point, some court will just decide that the email body constitutes meta data

Technically speaking, they are part of the mail header along with metadata such as date, recipient, priority etc.