Well, in my area of the world track shorts are the most common thing girls wear in warm weather.
But I can see why bike shorts are common
-Breathable and comfortable (like track shorts)
-Tight, so they show off her legs and give nice curvature to the ass.
Leggings, yoga pants and track shorts also look funny and weird when they aren't worn with other similar pieces of clothing or for the right purpose (those girls don't do yoga). However, that isn't the point. The point is that they are all comfortable and can add some better looks to the legs and ass, something many girls are insecure about. Also, many girls enjoy male attention, even if it isn't "intended" or if it's the wrong kind of attention.
Maybe for the increased attention... probably to somehow signify that they aren't wearing underwear. Because apparently, wearing underwear with bike shorts for women is REALLY dumb and will just chafe the hell out of you (says google link )
Or maybe they're just doing it to stay cooler, who the hell knows. I'm not an expert on clothes, just logic lol