Because they come to costco in the first place to... get ready for it... save money.Fair enough...
Costco's clientele is not the minimum wage earner - or anywhere close to that. Last that I checked, their memberships are $60/yr. Minimum wage earners are usually not Costco members. So, why should Costco be obligated to cater their food court prices to that audience?
It's not an obligation though, they can charge $5000 for a sammich if that's what they want to do.