We already live in an anarcho-capitalist society. It's just that in the U.S., there is a government that has managed to convince most of the others in the area to acquiesce to it. Those few alternate governments that do arise aren't able to convince the U.S. Government and its subsidiaries to choose a different arbitrator than the ones they currently go to.
If you can explain how to construct an Anarcho-capitalist society that somehow prevented a sufficiently militarily strong government from deciding not try to dominate or collude with all other governments operating in its territory, I would love to hear it.
Not always, take a look at tragedy of the commons:
https://en.wikipedia.org/wiki/Tragedy_of_the_commons