Because food is chaos. You are essentially packing a mini picnic at 35,000 feet. The air is dry. The seats are cramped. But you have a ziplock bag of homemade cookies. You are a hero to your seatmate. You are a rebel with a crust.
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Also, airplane food is weird. Your taste buds change at altitude. Sweet things taste sweeter. Salt tastes dull. That’s why airline tomato juice (yes, tomato juice) is wildly popular up there. It’s the perfect savory bomb. Bring your own snacks, and you control the flavor dimension.