If that "defense" is actually aggressing against people, they're pitting themselves against the entire market for actual defense from such things. Not a lucrative business model.
Modern war only exists due to the tax streams to plunder. In the absence of taxation, war is too costly (and just awful), not to mention crippling to the reputation of a business (nobody wants to subscribe to a security company that puts the customer in danger by picking fights).
RE: The Mathematical Case for Anarcho-Capitalism