Profit maximization. You make the point that companies can choose to hire employees that aren’t the bottom of the barrel but then you’re selling a different product to those who really are squeezing every penny. CostCo and Walmart are both successful superstores but one sells many more products, in smaller minimum purchases, and hires lower quality employees while paying less. These kinds of differentiation factors lead to what’s called oligopolistic competition[1]. They’re not in separate markets but they’re it selling the same product either.

[1] https://www.monash.edu/business/marketing/marketing-dictiona...