First, Mulan first came from a play by a gifted scholar in the Ming Dynasty, in which Mulan's surname was Hua. Then the script was used by the troupe, adapted into a drama and performed in various places, and spread again and again, widely known and influential, so the name Mulan came into being.
Secondly, there are many stories about Mulan, with different dynasties and surnames, so Mulan is probably a collection of images of many women who joined the army for their fathers. Some people say that Mulan's surname is Zhu, and she is from Songzhou, while the Yuan Dynasty document "Argument on the Filial Piety of General" also says that Mulan's surname is Wei, and she is from Bo County, Anhui Province. There may be many women who served as soldiers for their fathers and brothers in history, but the recorded Mulan is actually the representative of many women who joined the army, and with the spread of folklore, the image of Mulan who doesn't want men is more widely known.
Thirdly, from a realistic point of view, women's physique is weaker than that of men, and there is a physiological period every month. If there is no independent tent in the army, it is difficult to hide their gender. Coupled with camping in the military camp, it must be more bitter than living at home. If a woman is a woman, few people can adapt to ancient military life.
Fourth, in ancient times, you had to check the state of your army. If you don't check your hukou, the age difference is too big. Even if you put on makeup to cover up the difference, it's not a fragment, so it's hard to make do. Ancient people can't even tell men from women, can they? Unmarried girls and mature old men are still very different.
The above is my opinion, I hope to adopt it, thank you.