Why are wireless headphones so rare?

Almost everyone I see using a portable music device is using headphones tethered to the device by a wire connection. I use them too, but the wire is a pain in the butt to say the least and constantly gets tangled. This must happen to others as well and there are wireless headphones out there that aren't outrageously expensive and get at least decent reviews soundwise (I just checked this out). I'm probably going this route but just wondering why the wireless 'phones seem so rare out there? Why do people keep putting up with the hassle of wires? Is it just a lack of knowledge about wireless headphones? Anybody have experience using these products? Thanks for thoughts.