Why are factory workers, restaurant waiters and supermarket cashiers all poor, and these jobs are only done by the poor?

The factory workers, restaurant waiters, supermarket cashiers, etc. you refer to have low professional ability requirements, so most people who do these jobs lack stronger competitiveness. Their jobs are easily replaced by others. So their salary is not high.

As for your poverty, it is only the cumulative result of various factors, not the cause!

It's not that the product manager is rich, but he may be heavily in debt.

It is not the poor who are engaged in low-paying jobs. It is the lack of natural resources that makes them unable to master stronger competitiveness, which leads them to do less demanding work.