With a product such as Skype, it is almost a "standard" in that it is recognised, and used by a lot of people, across many devices. (Skype also adds video to the call, something that the majority of SIP calls don't.) Same thing with twitter, Facebook, You-tube, and many others... it makes sense that they are integrated into many of the products we use, such as smart boxes, or TV's themselves. People are familiar with them, and use them on many types of devices. Not so much with SIP. Sip requires that you are using (establish an account with) a VoIP provider, and in many cases, some technical knowledge in setting up the device. If a VoIP provider such as Vonage, were as widely accepted (used) worldwide, then you might see a SIP application built into TV sets. Even as large as they are, they are still a minor player in the big scheme of things, and like many VoIP providers, they don't want you "messing" with the settings on your hardware. Add to this the fact that, for residential customers, the use of land-lines, in general, is in decline, I wouldn't be holding my breath.
If there is money to be made, integrating a SIP client to a TV, then it will happen, otherwise.....