The reason ALDI pays employees really well is because the entire store is run with only a few employees at a time. A significant portion of the value you get from the store (cheap prices) is because their labor costs are low. I'd go as far to say that the real thing you are buying at ALDI is products that don't have a lot of labor waste built into the price. I've heard most places have 2 to 4 working at a time, but I've seen stores where a single employee is running everything.

Every employee I've seen there is working very hard the entire time. Before they added scanners, they were required to memorize the price of every item in the store. They pay more because they will not get the type of employee they need by paying less. While most of the people who are a good fit for ALDI like it there, you can find many reviews saying the work was too hard and that there was too much pressure.

If you look at retired RVers who have written about working at Amazon.com, you'll find many who felt that while the work was hard, it was good work and paid well (especially with holiday overtime) but you had to be willing (and physically capable) of doing hard work.

Like you say, the cost of each individual purchase from a company has little to do with how much they pay their employees. There is a much stronger correlation between employees who are doing hard things that add significant value and higher pay.