Perhaps because there's less at stake? If you hire or refer a close friend and then you become coworkers, the nature of the work relationship could negatively

Perhaps because there's less at stake? If you hire or refer a close friend and then you become coworkers, the nature of the work relationship could negatively affect the friendship, and people may not want to risk that if it's a friendship that they really value. If they're just an acquaintance it's not such a big deal.