Why are there so many corporations?

In a free society I would expect many different models of partnerships/companies, but in a statist society of today we see a lot of corporations instead, which is only a very specific model of enterprise. What drives people to create corporations then? Some say its a state privilege, but I don’t think it is. See Kinsella here: http://www.libertarianstandard.com/2011/10/18/corporate-personhood-limited-liability-and-double-taxation/

So why are there so many corporations? Is it more difficult to create partnerships with special contracts? What disadvantages does it have?

Kinsella says that double taxation is not a benefit because the corporation is being taxed twiced. But who wouldn’t rather be taxed multiple times if the total amount paid in taxes is less than being taxed only once. (This is possible because the tax rates are lower.)

EDIT: Although I do agree with him that some people think that they can get away with murder if they have limited liabilty; there is always “piercing the corporate veil.”