Without a single entity, a powerful government, breathing down their neck, your multiple corporations would merge or form a cartel, because they can make more money that way.
That's debatable, but it's not an argument against having government and business and non-profits and individuals able to keep each other in check. Again, competition and having organizations with cross-purposes having to work together to peacefully co-exist seems to result in better outcomes for most people. Again, nothing is perfect. But, unchecked power is scary, and a world with only one powerful organization (whether it be government, the mafia, a corporation, or a church) would be a terrifying one (and history provides many examples).
Also note: I have, nowhere in this thread, suggested the wholesale destruction of government and replacing it with corporatism (or whatever you want to call it). I have opinions on those subjects, but I'd rather we stay on-topic.