It's a matter of preference, style, and fit. It may be too hot to wear another layer of clothes. Maybe a woman wants to be extra modest just in case. Maybe shorts are too noticeable under a certain skirt or dress, so it's not added. There are different reasons.
Some girls find it more comfortable and some do it to avoid flashes of underwear - It is really one of those personal choice things - Maybe in guy terms why some guys avoid particularly tight trousers.
I wear shorts or a spanx kinda thing under dresses & skirts because my thighs are thick & close together. If I can't wear something, I'm stuck carrying a travel baby powder bottle to keep from chafing lol...
Depends on the comfort of a girl's style. If she barely wears dresses/skirts, she would wear some. If a girl doesn't it's probably that type of girl that's professionally handles them at least wearing them a few times a month. I personally wear biking shorts since I can't sand them. Even if the dress or skirt reaches my feet.
It's a preference. I know many girls who feel uncomfortable without shorts on under a dress because they feel over exposed. I choose not to wear shorts under dresses cause I don't feel like there's any reason to, even on windy days. I know how to not show my butt.
Personally it depends on the day and if it's windy, and how short it is. But I hate wearing shorts it defeats the purpose. I think it jusy has to do with caution more than anything. Some girls are more cautious than others.
Some girls just don't want the attention or get tired of it. It might not be their style either. Personally I don't wear short skirts or dressed because it's uncomfortable and not my style. I have friends who wear those clothes for confidence. While usually, they are very insecure.