Maybe because people without jobs can't buy things.
Henry Ford understood that by paying his employees well that he was not only incentivizing people to stick around to learn the skills to make good cars but also creating a future market for his products.
Noted Nazi sympathizer Henry Ford may have realized that, but he also thought he could help reshape society in his own image. Also, TPTB don't seem to realize that now. Wages haven't kept up with the cost of living for decades, let alone worker productivity.
I suspect that there's one thing a robot will never be able to do, at least not for a very very long time. A robot cannot create more humans.
The people at the top of the ziggurat believe that it's undesirable to have so many humans. As part of their own creation myth, they believe that the primary thing that's special about themselves is that they show up every day and apply themselves in a constructive way. Hence the rich guy in the news now for trying to prove that and winding up with health problems and abandoning the experiment instead. Who succeeds is much more a factor of who is lucky, as the most reliable predictor of economic success is the economic success of the parents, and we don't choose them.
That we still need a vast variety of humans to have a creative and developing future won't deter them from trying to depopulate us. They don't understand why that don't work specifically because understanding why it won't would require that they understand that they are special primarily in starting position on the board, and not in something inherent to them.