Costco’s customers are the top 3 socioeconomic quintiles, Walmart’s customers are the bottom 3 (or middle 3).
Either way, Costco’s customers are richer, and they have a different business model. They are objectively a good employer, but a lot of that has to do with their customer not being extremely price sensitive and being able to afford to buy large quantities, which also results in less loss due to theft.
You can find the richer parts of any town by searching for Costco.
As for founder net worth, it’s not very meaningful if a founder owns it or others own it in 401k. Divide the Walmart founder net worth by employees and it still would not give them a meaningful improvement in wages for long. They’re simply a low margin business selling to customers who cannot afford to spend extra.