Most everyone here has a businesses, either in the formulation or already off to the races, so wouldn't one have to give up theirs to co-found with the other?
It may seem that everyone is already committed to a start-up, but that's not always the case. I am actually looking for my next co-founder at this time (feel free to connect w/me offline). By definition, a start-up is a temporary organization that's looking for a working business model. Since majority of start-ups do not reach that milestone, and they have a limit on how long they have to do it, most [early stage] entrepreneurs are likely to be looking for their next team/opportunity every few months. So, take heart, keep looking, and continue to stay in the game. It will happen, if you have the patience and commitment to your goals. Good luck!