It's about number of people who can take risk of having zero income for N months. That is such a small group of people, and this narrows the pool of people who might found a startup that could change the world (or part of it). It means that all the ideas for startups are coming from people who have (at least) one thing in common. Maybe it's not the optimal thing to have in common?
ps. I did the startup thing with a wife and a daughter. These days you know it as Amazon.