Because the "eight characters" are determined by the dry calendar and expressed by heavenly stems and earthly branches, while heavenly stems and earthly branches has five elements, which makes the "eight characters" also have five elements, which correspond to everything, so everything can be expressed by five elements, such as the son (mouse) belongs to the sun and the north; Hai (pig) belongs to yin water, north. Yin (tiger) belongs to poplar, oriental; Wool (rabbit) belongs to the shade wood, oriental. Noon (horse) belongs to yang fire, south; The fourth (snake) is yin fire, south. Shen (monkey) genus, western regions; You (chicken) belongs to Yin Jin, the Western Region. Chen (dragon) and Xu (dog) belong to the yang soil, in the middle; Ugly (cattle) not (sheep) territory, center.
The traditional naming method has little to do with the eight characters, but mainly with ancient books. The names of many celebrities are extracted from ancient classic books. For example, the name of Tu Youyou, the first Nobel Prize winner in medicine in China, is based on the famous sentence "You Luming Literature, an apple in the wild" in The Book of Songs Xiaoya. Another example is Luo, a "four outstanding men in the early Tang Dynasty", whose name and ideogram come from the divination in the Book of Changes: "Looking at the light of the country, taking the guest as king". Later, the folk custom of "eight characters" prevailed, and some people used "name" to make up for the lack of eight characters. In fact, there is no theoretical basis, at least there are few such records in historical sites.
Even if you choose a name according to the number of five elements of the eight characters, it takes years, months, days and hours. Obviously, it doesn't conform to the viewpoint of eight-character naming, so don't entangle these folk customs. After all, folk customs are just legends and have no scientific basis. There is no point in clinging to them!