Without government regulation, companies hurt people (e.g. unsafe working conditions, denial of health benefits, toxic dumping, unsafe oil rigs etc...).
They make harmful products (e.g. dangrous drugs, cars that blow up etc...).
They treat employees horribly (e.g. discrimination, wrongful term, etc...).
And no - those companies don't disappear if they are bad because "the Magical Market Corrects All".
The Market does little to correct anything a company does, once it gets big enough. That's just plain fact.
So the biggest flaw I find in Libertarianism is the belief that companies will regulate themselves, if simply left alone. History proves this is not the case.
Nobody is forced to work at a company with unsafe working conditions. Pollution is a failure of government and a lack of private property. The most polluted areas of the environment are the water and the air, both of which lack property rights and are essentially considered owned by government. Discrimination will occur with or without government if that is the morality of the people. You cannnot regulate morality.
The irony is that if there are so many people in the market willing to do all these terrible things, what type of government officials do you think will gain power? The absurdity is making the general argument that humans act immorally in a marketplace and we thus need to put those same humans in a government where there is even less accountability and actions are coercive and not voluntary.
Furthermore, the market is not magical, nor is it perfect. No creation of humanity can ever be perfect because human beings are imperfect in nature. And the belief that companies will regulate themselves is simply not a libertarian belief. Companies are regulated by the free market. By profit and loss and consumer action.
This is why a strong centralized government and reasonable level of regulation is necessary to the well-being of citizenry.
Essentially you are saying that an institution with a monopoly on violence that obtains funds through coercion and prevents voluntary action between willing individuals is necessary to the well-being of citizenry. You only need to understand what government is, how it is supported, and how it accomplishes goals (the means it uses to achieve its ends) to see how it disrupts well-being.