They were all dependent on luck and outside circumstances. Without Bill's mother's connections that got him in the door at IBM, Microsoft may've never been anything more than a blip. Without the gross indifference to search pre-Google (Y! focused on being a "web portal", not search results), Page/Brin may've just been starving grad students that eventually found a semi-OK job in academia. Most blatantly, if Jobs didn't have a passive engineer like Woz to co-opt, he would've been just another self-centered burnout.