Airports do solicit competitive bids for concessions, they just end up awarding it to a single concessionaire. My local international airport is owned by a regional governmental entity, one of two in the United States (Metropolitan Airport Commission in MN and MSP Airport, part of the Metropolitan Council which encompasses a 7-county region. Portland, OR has a similar regional government org but I am unsure if theirs owns PDX) and all RFPs for contracts over I believe $100,000.00 are visible online. [0]

Keep in mind that the venue owner has overhead for each and every vendor, awarding a concessions contract to a single entity simplifies things for the organization that owns the airport. There’s also a limited amount of space in an airport and having one concessions company storing food for all of their ‘restaurants’ in one location instead of having a dozen or more tiny food storage areas simplifies things for the airport yet again.

[0] https://www.mspairport.com/business-at-msp/business-opportun...