Let's say that you owned a widget making company, and virtually every industry had become as automated as possible, to the point that there were so few jobs, there were not enough consumers with money in their pockets to keep the economy going.
So why would you decide to suddenly start replacing automation with human labor? That wouldn't do you a bit of good, because there are still too few consumers to keep your widget factory going. And where would you come up with the money to hire people? And do you really think that just your employees alone would purchase enough of the widgets that you made to keep themselves employed and still allow you to make a profit?
It's a dilemma with no private sector solution.