Because here is the definition of capitalism:

Capitalism is a social system based on the recognition of individual rights, including property rights, in which all property is privately owned. (Credit goes to the Ayn Rand Lexicon for this particular wording of the definition)

No form of anarchism can protect individual rights, therefore, anarcho-capitalism is not capitalism.

Except the scenario that _my_ gang, also known at the U.S. government, happens to expand its territory to cover the entire United States, which is the goal, and enforces a monopoly on government.