The amount of work expands to fill the available labour. All other things being equal, at least. Which they aren't, but it's a usefully wrong model.

There’s many praises to sing about efficiency, (and I don’t take your 1 liner as a position against it). That said, efficiency, job creation, and underemployment overlap quite a bit.

There’s far more scientists, programmers, and doctors today than farmers and stablehands.

At the same time, people who lost manufacturing jobs to automation and outsourcing, did not get jobs with equivalent pay and growth.

Human brains do not get retrained very easily, and so every technological revolution is a boon to those who grasp it, and a challenge for those who invested their time in skills no longer in demand.