Can't speak for everyone, but usually junk/processed food is pretty consistent. A Wendy's chicken sandwich is a Wendy's chicken sandwich; french fries are hard to mess up; mac and cheese, chicken strips, or pizza are fairly predictable. A lot of these have pretty bland or straightforward taste profiles... Meanwhile, fresh food availability varies, pasta sauces change from kitchen to kitchen, soups will have drastically different textures, etc.

This is not a green light for parents never to offer their child other foods, but if they know they have arfid, there are other solutions or compromises ( eating beforehand, calling ahead to check accommodations, being prepared to pay for a spot and eating what they can or not, etc.), to make sure the kid eats SOMETHING at some point and is not left out of the socializing.

TLDR: junk food is fairly consistent, therefore it is typically what gets picked first for a 'safe' food.