The main reason is that future generations have no education. Yue Fei belongs to the hero in folklore. In ancient times, no one dared to call Yue Fei, but they were all called Marshal Yue, who had a higher status than Yue and was respected. Just like Guan Yu and Master Guan. When the ancients mentioned Guan Yu, they were all called Guan Yunchang. When our generation has less culture, it will be called Yue Fei directly. Yue Fei and Guan Yu are ancient folk gods.
When the ancients called people by their first names, they could not call them by their first names. This is not only respect, but also a habit and rule. For example, Ma Chao wrote to Liu Bei: "Meng De killed my whole family." After killing his whole family, he still has to call Cao Mengde, otherwise it will look ridiculous and tasteless. In modern times, second-rate scholars and third-rate screenwriters popularized ancient culture to naming. When filming a historical drama, Lin Zexu's wife called him: "Zexu." Really, Lin Zexu can slap his wife. Lin Zexu's wife should call him "Fu Yuan" all her life.
Until today, it is not convenient for us to call people by their first names. There seems to be an order, an unfamiliar tone.