I don't think that's true. Physical motion and even sound making in total involves a human being producing a lot more than a few bits of information per second - there are plenty of recordings of people singing, say, that can't compressed to that bitrate.
The only thing that's true is that we can pretty much only emit and absorb language information at a fairly low bitrate.
The simple explanation for this is that a language stream is something like a computer program - difficult to either produce or absorb.
And of course, language with complex structures is fairly unique to humans and provides some amazing things - the ability to help organize society and our own activity on a larger scale, to plan ahead, and etc, and etc, etc....
Edit: Good general discussion of these issues can be found in Tor Norretranders' book The User Illusion. Highly recommended.