Ah, but there is a difference between a democracy and a stock corporation.
There are many differences:
1) If you don't like how a stock corporation conducts itself, you're free to dump their stock and refuse to do business with them. A democracy, on the other hand, will cheerfully commit horrible atrocities in your name and on your dime, whether you like it or not.
2) If you try to compete with a stock corporation, you may or may not be financially successful, depending on the circumstances. However, if you try to compete with a democracy (for example, in the areas of security provision, delivery of first-class mail, issuance of currency, etc.), you will be forcefully shut down.
3) The agents of a democracy can do whatever they like to you, usually without consequence. Between police and military forces, they murder thousands of innocent people per year, with no more than an "oops, our bad", and usually not even that. The agents of a typical stock corporation, on the other hand, are held to a roughly similar standard of behavior as the rest of us.
I could go on, but hopefully you get the idea.
Plus, how does the power to regulate make the government a monopoly? It's derived in law, not in bank account. If the government took on the project itself and refused to allow competition, you might have a point. But we have the opposite situation here: monopoly status is indirectly or directly conferred upon otherwise normal companies, as is done with the USPS, and people are saying, why bother "regulating" if it just results in special treatment and handouts? Why not actually regulate, so that there is some semblance of competition, which is supposed to be a main benefit of privatization?
A government is a monopoly, by definition. As long as a government exists, it will be disproportionately influenced by those who control large amounts of capital, and will provide them with special privileges at the expense of the rest of us. In terms of regulation, that means it's going to regulate in such a manner as to provide monopoly-level profits to its favored parties, intervening against them only where needed to keep the system stable.
The general public are akin to cattle, with the government and big business cooperating in order to milk us most efficiently. Our interests are secondary to this. The solution, then, lies not in calling for more (or "better") regulation, but in abolishing the government entirely.