Most girls seem to wear black shorts or spandex under their skirts and dresses :(

There are a number of possible reasons, and sometimes it can depend on their age. Comfort, a more slender appearance, and modesty are the most common reasons.

Its to keep pervs from looking up their skirts. And its them who seem to be the ones that notice or care.

No, I don't think so. You have been looking much to closely though, trying to see what is underneath skirts and dresses ! If women wear spandex at all, it is because they want their figure to look more trim. Spandex holds all the bumps in. Young girls today, as vain as some are, would want to wear the most lacy underwear.

They wear what they like and my question is why are you curious about it?

If you're a Kindergarten teacher, you wear slip shorts underneath dresses. Regular slips don't prevent little hands from lifting them up. Slip shorts are a lifesaver!