Why don't airlines cover their planes in ads?

An aircraft is not a billboard: any material installed in the cabin, from vinyl on a tray table to a sticker on an overhead bin, must pass the flammability, smoke, and toxicity tests required by the FAA and EASA under transport aircraft regulations (Part 25 and CS-25). Every modification requires documentation, approval, and sometimes a weight-and-balance review. That turns a campaign that on a bus is wrapped up in an afternoon into an engineering project with its own cost and timeline, which explains why what little advertising exists is limited to previously approved surfaces.

Airlines invest millions in making their cabin convey order, cleanliness, and calm; a surface covered in third-party ads contradicts that message and, according to the satisfaction surveys the airlines themselves run, irritates a passenger who is already stuck on board with no way to leave. Moreover, an airline's preferred advertisers are its own products: loyalty cards, onboard sales, upgrades, and car rentals, which generate more ancillary revenue than any third party.

Painting an aircraft with an advertising livery costs several days grounded in a hangar, the paint adds weight and therefore fuel burn, and the contract must cover that lost revenue. That's why special liveries exist (tourist destinations, films, sports sponsorships) but they're counted in units and are usually mutual-image deals, not advertising in the usual sense. Compared to the revenue from a single full flight, what an advertiser is willing to pay for the fuselage rarely offsets the hassle.