There's a principle in economics known as the "discipline of constant dealings", which basically holds that it's more advantageous to do right by customers and competing firms because to do otherwise is less profitable in the long term.

Others mentioned the cost of violence, but that can't really be overstated. The costs of maintaining a military are so extraordinary that only an entity imposing compulsory taxes on an enormous number of people could afford to do it.

Maybe Apple, with its $700B+ market cap, could raise a small army, but war and violence have uncertain outcomes — much less certain than the profits to be gained by developing products people want to buy. Developing compelling products may not be easy or inexpensive, but it's far easier and far less expensive than violence, and also has the benefit of a far more certain outcome.

It seems unlikely that without the support of a corporatist government, firms would grow to the size which they have today. Without copyrights and patents guaranteed by the coercive power of the state, and without subsidies and government-guaranteed loans, it seems unlikely that firms such as GE would grow to their current size in the face of competition.

In the absence of a state, local militia and rights enforcement agencies would be developed to protect people against coercive agents.