Some anarchists are opposed to all hierarchical institutions, but not all anarchists are the same. Like I said, anarchism in and of itself is merely defined as opposition to the existence of the state. So anarcho-capitalists, being opposed to the existence of the state, are definitely anarchists.
Why don't anarchists like the state?
Depends on the type of anarchist we're discussing. Most, if not all, anarchists would answer that the state has proven itself to be one of the greatest mass-murdering entities in the history of the world. Which is valid. An anarcho-capitalist might respond that the state is institutionalized aggression, and that everything it does constitutes force which the anarcho-capitalist sees as illegitimate. Anarcho-communists might respond that the state props up capitalism, and that minus the state capitalism's inherent flaws would cause its collapse and bring about some kind of communist utopia.
And what does the government have that let's it get away with what anarchists despise?