When you consider that there is no limit to the size of a company it is pretty obvious why monopolies form. A lot of markets only really need one company that provides a service or product for the entire economy. With healthcare it is extremely obvious that having one provider is beneficial because you want everyone to be on the same health insurance plan. The problems are that monopolies usually abuse their power so you need put it in control of an accountable entity such as the government.

Consider it from a different perspective: CEOs don't become billionaires because they are multiple times more productive or competent than other people but rather because there is no need for a second equally competent CEO. One CEO is enough to serve an entire market and so all the market power concentrates in the hand of the CEO who can then abuse it for his benefit.