Alignment of incentives. I'm sure the personal humiliation of being yelled at by Jobs was a reasonably strong incentive, but I'm certain the perception that failing to deliver would have him personally sending you to the dole queue asap was even more of a strong incentive.

Compare to most corporations where the only thing you can do to get fired is fail at office politics and failure to deliver/delivering the lowest quality crap that can be passed off is just business as usual.