Actually the cable infrastructure was built almost exclusively with private money. Most of the grants and taxpayer money were to support universal access (i.e. paying the cable companies to wire up the less profitable areas of town). They wouldn't have even needed them in the current deregulated telecom environment.
Regardless, the build-out happened almost 40 years ago, and the infrastructure that exists today is a lot more complex and would cost a lot more to deploy today. Almost all of that complexity has to do with data transmission, which is a product that was never envisioned when the cable buildout was happening.
