First, the setting of immortality is "harmful"
In the demon world of Journey to the West, there has always been a rumor that you can live forever with a bite of Tang monk's meat. Moreover, "steaming" can ensure the original flavor of food to the greatest extent without losing nutrition, so the little fairies think steaming is the most reliable way to live forever.
Then there is the story of Journey to the West, which took place in the Tang Dynasty. There were only three cooking methods in the Tang Dynasty: steaming, boiling and baking. Cooking only appeared in the Song Dynasty, because it was not until the Song Dynasty that iron pots were widely used. There are also many questions about why monsters don't eat directly. Maybe they think eating directly is parasitic. After all, the monsters in Journey to the West still know how to keep in good health.
Second, Wu Cheng'en is keen on "steaming"
Wu Cheng'en's hometown is today's Huai 'an, where Huaiyang cuisine is the most famous, and steaming is one of the main cooking methods of Huaiyang cuisine. Many historical celebrities like this kind of steamed food that keeps the original flavor of the ingredients.
For example, at the end of the Ming Dynasty, Li Yu wrote a book "Follow the Fate", which greatly praised steamed food: "It can make fresh fat generate innocent, suitable for Yu Xian in the evening, and the steaming is wonderful." When Mr. Wu Cheng'en was writing about how to eat Tang Priest in The Journey to the West, his lips and teeth could not help recalling the delicious steamed food in his hometown. Therefore, he should firmly believe that steaming Tang Priest is the most reasonable thing to pursue the demon.