I think there's a practical consideration to the single-founder limitation.

Anecdotally, I'm running my own business now. Actually, it's running me, and that's a problem. I really wish I had a partner, not just to take some of the load off of my shoulders, but to motivate me when I get discouraged, and so that I didn't have to be the only one coming up with all the ideas.

Also, single-founderism might indicate that a person is less sociable, or difficult to deal with, or (as in my case) determined to do it alone. Those are all rather dangerous things for a startup.