The buttonfly, the opening on the front rise of a pair of jeans, like these @levis
501s, that fastens with buttons below the waistband was, at one point, the only way to climb into and out of your denim.
Sturdy stud buttons made of two parts: a short nail pressed through the fabric, and the visible head made of copper, brass, aluminum, or stainless steel are less likely than a zip fly to break, catch, or come undone.
This traditional and dependable feature predates the advent of the zipper and, while they can create more bulk, they create interesting fade patterns on the front rise.
The cheaper zip fly is linked to the advent of the denim sadforization as fabric stabilization is essential to zipper performance as the length cannot adapt with shrinkage.
Go old school my friend and stay buttoned up.