May be due to the following reasons! 1. Transformers belong to the United States, which is too far from Chang 'an. Tang Priest went to the Western Heaven, which is today's India. India is much closer than America. It has been so many years, it may take him a lifetime to collect a Transformers in America. What else to learn? The Tang Priest has been away from us for more than one thousand years. He has been translating scriptures in Xi 'an for many years, and Transformers is an American. The history of the United States is only a few hundred years, which means that the Tang Priest has ascended to heaven. Transformers haven't come to earth yet!
Second, those three apprentices are enough. There are many people, high labor costs and more food. Not many people, just enough!