There's a few things stopping mercenary corporations from taking over the world. Chief of which is that, at the end of the day, your standard government is simply a tax-funded security firm and mercenary organization. The only difference is that governments want power more than they want money. And governments are generally publicly owned.
Arguably, by the time a private company outguns the government, they ARE the government. It would just be funded slightly differently. Maybe. It's possible that such a company might just decide to fund everybody who uses your services. Oh and for PR reasons you might try to have people decide the terms of use through voting or something. And then you're back to square one.