It's not that a rooster can't make soup, but that a rooster is not suitable for stewing soup.

When stewing chicken soup, people prefer to stew it with hens, because the cock's meat is more delicate than the hen's. If you stew the soup, the soup often looks clear and watery and not delicious. The melting point of hen's fat is relatively low, so it is easy to stew hen's fat when stewing chicken soup, and the stewed chicken soup will be more delicious and nutritious.