You seem to be asking for telephone calls and SMS messages to be end-to-end encrypted. The underlying technologies were not designed with encryption in mind, so the only way for it to work would be for all the participants in a conversation to use an additional software layer. That was the method used by TextSecure.
The authors of TextSecure eventually figured out that a purpose-built Internet-based messaging protocol would be a better transport layer for secure messaging. If you're interested enough in secure messaging to be asking this question, you may be familiar with TextSecure's successor.
As for why a carrier wouldn't do this, I'll ask the inverse: why would they put in the effort when anyone who cares about secure communication just uses an encrypted messaging app?