They were created from scratch in the mid-1970's after the launch of HBO by what was then Time, Inc.. HBO created a new source of programming for cable operators which stimulated them to purchase satellite receivers. Other companies, and a few entrepreneurs like Ted Turner, saw an opportunity, leased satellite transponders like HBO had done (at first on the same satellite), and began delivering programming to cable operators.
There are no "empty" networks especially these days. It's an expensive proposition to obtain programming, lease transponders, and market the service to cable operators. We're talking about investing millions of dollars before seeing a dime in revenue. Outside of large media conglomerates, there are few individuals who can command the resources to launch a network.
Mark Cuban took the plunge with his HDNet services a few years' ago, but he's a billionaire.
Terrestrial broadcasting networks have an entirely different history that dates back to the introduction of television in the late 1940's. Individual entrepreneurs obtained broadcast licenses for the initial 3-4 VHF channels allocated to their markets, then worked with the existing radio networks (NBC, CBS) to develop programming. The existence of radio networks, and their established relationships with advertisers, made this feasible.