"Fly" does not refer to the button, specifically. "Fly" refers to the flap of material covering the opening or fastening of a garment. read more
The region which is tied up either by the buttons or zipper is named as Fly like zipper fly with zip and button fly with buttons. You misunderstood that term. It actually your choice which trousers you like to buy either with zipper fly or button fly. read more
It's style. I still like the classic Levi jeans with a button fly. I can understand problems with urgency but you can always find trousers with zippers. What I hate is boxer shorts with button flies. read more
Notion of "flapping as a wing does" led to noun sense of "tent flap" (1810), which yielded (1844) "covering for buttons that close up a garment." Wikipedia says: In the 1930s, a sales campaign began for children's clothing featuring zippers. read more