Why don't women have Cat. 5? 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?
Its to control the size of the field. The number of categories grows as the number of riders increases. This is so that you split people by ability more (for better consistency) and keep the fields sufficiently small. 

There just happen to be more men than women. If the number of women increase to be unwieldly with 4 categories, they'll presumably add a cat 5 for women as well.