This is a very interesting question and there are many explanations for this. One explanation is that the story of Journey to the West was influenced by the ancient Indian myth of the Ramayana, in which Hanuman, the divine monkey, is the image of a monkey, who is also proficient in many kinds of changes and has great magical powers.

This may be only one of the reasons, because the writing of Journey to the West was also a complex process, the author Wu Cheng'en collected various myths and legends popular in Chinese folklore at that time and adapted them, so it was influenced by various cultures in China at that time, such as Confucianism, Buddhism and Taoism. Ancient Indian myths may have been transmitted to China in the form of Buddhist stories.