I'm a founder who is pretty sympathetic to the idea of cooperatives.

The simple answer is that I am not savvy enough with financial instruments to be confident that I can structure something that can't be preyed upon if it's successful. I know I care about making sure my employees are well-treated - you shouldn't believe me when I say that, but I believe me, and that means that in terms of my own ethics there's not much reason not to maintain control and depend on my own judgment. If I want to divide 70% of the profits up among my employees, nothing is going to stop me from doing that, so why not keep my options open?