Some people DO work 16 hours straight if they don't want to get fired. I worked for a guy who did exactly that on an important project for GE.
Of course, GE also paid him handsomely. Not because GE is a benevolent organization, but because he would have quit. It's the same reason why Henry Ford paid his workers so well--he had one of the highest turnover rates among big manufacturers.
Actually no it isn't. Throughout history, slaves are good at one thing: basic agricultural work. And even then, it tends to disappear because of enforcement costs--it disappeared in 18 or so different countries in the americas, without firing a shot.
Oh and also there's this: