Using TLS assumes that the next hop in the SIP proxy chain can be trusted and it will take care about the security requirements of the request.
This helps to get the critical mass of implementation to make this method the de facto standard.
If you use a trusted intermediate for this, the call setup delay will significantly increase, which makes applications like push-to-talk difficult.
For example, your operator might implement a B2BUA architecture and play the role of the other side, so that you still don't have end-to-end security.
Newer, modern protocols, like ZRTP, offer end-to-end encryption for SIP/RTP calls.