Historically, it was one of the cheaper items on the menu. In college (20 years ago) I could get a large fries for under $1. Back in the 80s/90s they were practically free. Like, loose-change free.

Also one of the tastier meal items given that they went so fast you could safely assume they’d be fresh, why the sandwich options could be sitting in the warmer for half an hour or longer depending on the speed of business.