Are they, though? There are points well short of fascism where they exhibit significant similarity and cooperation. The idea of government as the remedy to every problem created by corporations is as overly simplistic as the idea of government ruining everything that corporations create. Either can be oppressive. Either can be beneficial. Some level of cooperation between the two can even be a good thing, as e.g. when corporations were first created to do things that were too expensive for one and too risky for the other. The similarities between organizations of similar size - also including churches, universities, and so on - overwhelm their differences. I would say corporations and governments are two wings of the same bird, and should remain that way, if the body in between is to be a healthy economy as part of a healthy society.

P.S. Yes, I'm well aware that the two exist for different purposes and answer to different kinds of authority. Don't even bother. The point is that their structure, behavior, and actual effect on society are not so dissimilar as you make them out to be.