Why is that when girls dress as guys it is not cross dressing?

Back in the 40's and 50's it was NOT socially acceptable for a girl to wear boys clothes (like trousers). Girls were not allowed to wear pants to school, or church, or at work, or any professional or public setting. Society said that those girls were bad. They were trying to act like men, and were a threat to femininity. However, girls continued to wear pants and other boys clothing. They did so without trying to act like men, and while maintaining their femininity. It took decades for society to accept this. That is why woman have earned to right to wear pants and other men's clothes.