No, idiots. It's because event promoters and club owners who set the rules are mostly men (Correct me if I'm wrong, and clubs and promoting companies are actually overwhelmingly headed by women). Men want to entice men to come to clubs by packing it with women. Men want to entice women to come to clubs by giving them free admission.
If the incentives were reversed, would the strategy be just as successful? For example, women are enticed to come to clubs that are packed with attractive young men. The young men, on the other hand, are enticed to come by the offer of free admission. I'd like to think that in a gender equal world, the strategy would have the same effect whether it was men or women who were given the free admission.
However, from my own personal experience, it seems like men are the ones who are requesting to be outnumbered by women. Most women I know don't care whether or not there's going to be a lot of men as long as there are "cute" men. Men I know tend to care about the numbers. They express revulsion at attending a "sausage fest" and whenever they throw parties, they specifically tell me to "bring girls."
So let's think about this. It's men who have decided that a great party is one where men are greatly outnumbered by women. It's men who run these promoting companies and clubs and set up these policies as part of their business strategies. It's the overall fantasy of being surrounded by beautiful women that fuels these enterprises and you, penis possessor, are getting all upset as if women are somehow responsible for manipulating club owners to gain some nominal advantage? Stupid.