For road racing in the US, USA Cycling has women divided into four categories (4,3,2,1) and men divided into five categories (5,4,3,2,1). The procedures for category upgrades for 4-3, 3-2 and 2-1 are the same for men and women, but only men have this extra "category 5", where the typical way to upgrade 5-4 is to simply finish a certain number of mass start races. Women don't have an equivalent "experience-only" category.
Why don't women in the US have a Category 5, like the men?