Consider also that in the past, many of the jobs shifts took place on a generational scale. A farmer--instead of having 10 children that were farmers or farmers' wives, and one that was a non-farmer--would have one child (or less) that became a farmer, and the rest would become non-farmers.

The economy changed slowly enough that you could have the same job your entire life and not be suddenly unemployed or unemployable when the technology changed.

After the industrial revolution, the pace of economic change accelerated. You can now make an entire job obsolete in a shorter period of time than the ~50 years it would take for a new worker to reach a socially acceptable retirement age. And what is that person going to do for a living? Can they afford to restart their lives from the second-lowest rung on the ladder?