You're viewing government as a benevolent force. The reality is that government as an organization is funded through coercion (taxes aren't voluntary). It also protects itself from competition through violence (try competing with any of the "services" provided by a government, the government will shut you down). It also uses violence to further its aims (if government wants your land to build a road, it can just take it (and in the US, if government takes your property, it is mandated by the Constitution to pay you)). A private entity can't do that.

In particular, as a user of cable, a private cable company cannot force you to accept and pay for its service. A government-operated cable service could.

From the perspective of a private cable company, government competition is unfair. The government does not have to use non-coercive means. It can force people to pay for its service even if they don't want to.

In the US, the best illustration of the danger of government enterprises is the public school system. Residents of an area are forced to support the local public schools, even if they send their children to private schools. The long-term loss of human potential of this system is monstrous. The economic waste is pretty bad too.

The best way to avoid disasters like the US public school system is to insist that transactions be voluntary. If you want to operate a cable company, fine, do it, but don't expect to be able to use coerced money to fund it or coercion to operate it.