Why is 果子 in 煎饼果子 (jiānbǐng guǒzi)?
"果子" is a regional name for "糖果糕點" - any sugar, rice or wheat flour-based food that is not a staple food. This broad category includes sweet and savoury items, cakes and candy.

The 果子 in 煎饼果子 refers to the 油条 wrapped inside. Of course, regional varieties of the food can have different fillings.