Why don't socialists support minarchism or anarcho-capitalism if it's the perfect system for them?

Socialists' biggest enemy isn't capitalism or the rich, it's the state.

Socialism is a post-capitalist ideology, not a anti-capitalist one, the state is the only thing that stops socialism to happen.

If you remove the state from capitalism, you can progressively move the society to one that is more socialist or communist.

It's that simple.