I don't have first hand experience, but my impression is that the best co-founders are those who you've got some history with, who you know and trust. That doesn't happen overnight.

Kind of like an actual dating site might be a good way to meet potential marriage partners, but you'd look at someone pretty weird if you went on a date and they suggested they want to get married right away.

But who knows; those guys see a lot of founders, maybe things work out differently.