Because in the real "classical Chinese era" (such as the pre-Qin and Han dynasties), there is no distinction between classical Chinese and vernacular Chinese. Even if the writer said it at that time, it would be written into words, such as historical records, Han books and hundreds of articles.
Language is constantly changing, and then what people say orally is gradually changing, but when writing articles and making sentences, scholars still deliberately adhere to the ancient writing method. In this way, there is a gradual difference between what is spoken and what is written. After the Tang and Song Dynasties, this difference has been very great. Some people write in spoken Chinese (such as Song and Yuan Dynasties), but more people are used to writing in the language of the ancients, because on the one hand, they show their knowledge, on the other hand, they like the rigor and conciseness of ancient Chinese, just like the upstairs saying, "Save paper and money."