There has to be a cable in the ground for the cable company to do this - you may remember 15 or so years ago when we all suffered with pavements dug up whilst the infrasstructure companies fought each other to install the infrastructure in our towns and cities.
Although BT has been forced to open up their infrastructure to competition from the local exchange (called Local Loop Unbundling), as far as I know they still have the monopoly on installing the copper cables from exchange to your house if you can't use a cable company network.