Why don't dancers do ballet in jeans?
Nowadays in the age of stretchy jeans, this possibility is highly likely...and some people DO, but not as many. Before asking why dancers don't wear jeans to dance ballet (jeans in fact is a product of the 16th century), let's think about why dancers wear tights or stockings to dance?
In fact, it’s quite simple. Wikipedia says that ballet came from the courts of Italy and France in the fifteenth to sixteenth centuries. Those years happened to be the era when knee breeches were hot and shiny among the nobles of the court (I heard those were really hot and people in that era didn't shower much). It's only natural that people started wearing tights when dancing ballet.
However, initially female were forbidden from performing Ballet and all female roles were played by male; of course there were also Ballets about love. Hmm...Maybe the BL element was too hot, or audiences were too excited when watching the performance of dancers' bromance, that their self-restrained of the court nobles had gone with the wind (editor notes: I was imagining two bearded rugged men wearing tights and looking each other right in the eyes...OMG TMI now I am blushing!). After the regulations were relaxed, allowing women to perform Ballet (Benevolent Rule!!), along with the introduction of en pointe technique, women thrived even more in this performing arts. Then based on women's attire, the breeches were changed into tights, and romantic tutus were worn on top of tights, and later the long flowy romantic tutus also developed into the shorter stiff and pointy classical tutus. According to the refinement of the aesthetics for Ballet over the years, modifications are made and thus become the Ballet attire nowadays.
History is history. But what if the nobleman at that time adored jeans instead of breeches, or that apron fisherman wore when slaughtering fish...it is hard to say how the ballet attire will look like today.