When Denim started, it was meant for men. Jeans were not widely worn by women until the 90’s, so I guess the originality is still associated with the good old Denim look. These days there are dozens of jean materials, that is why the stretchable versions do not come with the zipper. There are jeggings and other jeans that have elastic bands. So, I guess things might change over the years as well. There might be zipper-less jeans made in future!