Why should Dunhuang Mogao Grottoes be painted inside the cave instead of outside it?

Hello, the cave is well preserved, and it is not easy to be damaged by wind and sun, so it is easier to depict in the cave. The Mogao Grottoes were built in the pre-Qin period of the Sixteen Countries, and after the construction of the Sixteen Countries, the Northern Dynasties, Sui, Tang, Five Dynasties, Xixia and Yuan Dynasties, it has formed a huge scale, with 735 grottoes, 45,000 square meters of murals and 24 10 pieces of clay sculptures. Dunhuang murals include Dunhuang Mogao Grottoes, West Thousand Buddha Cave and Anxi Yulin Grottoes. There are 552 grottoes with over 50,000 square meters of frescoes. It is the cave group with the largest number of murals in China and even in the world, and its content is very rich. Dunhuang murals are the main part of Dunhuang art, with huge scale and exquisite skills. Dunhuang murals are rich in content. Like other religious arts, it is an art that describes the image, activities, relationship and relationship of God, so as to pin people's good wishes and appease people's hearts. Therefore, the style of mural painting is different from that of secular painting. However, any art originates from real life, and any art has its national tradition. Therefore, most of their forms come from the same artistic language and expression techniques and have the same national style. Dunhuang frescoes are also called Thousand Buddha Cave. It is also one of the four ancient caves in China, and is listed as the world intangible cultural heritage.