Because it is often hard to fill a podium with gender / age classes.

Same thing I've posted many times - no gender or age classes will work best. Every time I see a big race cater to the money loosing classes the next year, that race takes a break. See ToC this year. I've posted my list before.

Race everyone in grades 1-10 whatever they are. Those that race to get the prize, won't race. I'd rather not have them anyway.
Also repeating - no where near as unfair as junior's having gear limits when racing adults, but we ignore that. Folks tend to care about fairness when it affects them.