It is because they are indeed exaggerated figures in mythology, not because they do exist and are excellent leaders of ancient Chinese tribes.
This is related to the ancestor worship of China people. China always likes to talk about people who have great achievements or talents to worship myths. For example, Guan Yu became a warrior, a Dapeng, and a keeper with Qin. Bao Zheng became the underworld, and Lao Er simply became one of the Taoist Sanqing. They are all historical figures, and they are all mythical. The same is true of the three emperors and five emperors.
There are many sayings about the Three Emperors and Five Emperors. Among them, the most famous ones in Huang San are Nuwa, Shennong, Fuxi and Huangdi, Shennong and Fuxi, but there are others, such as The Biography of Shangshu, which regards Suiren, Fuxi and Shennong as "Huang San", as well as Custom Guide, White Tiger and Ancient History.
The Five Emperors are generally considered as Huangdi, Zhuan Xu, Di Ku, Yao and Shun. However, Shi Ben, Da Dai Li Ji and Historical Records of the Five Emperors list Huangdi, Zhuan Xu, Di Ku and Yao Shun as the five emperors. Shao Hao, Zhuan Xu, (Di Ku), Yao and Shun are regarded as five emperors in The Preface to Shangshu and The Century of Emperors.
The mythical Five Emperors are five corresponding to China's Five Elements, namely, Bai Di Shao Hao (gold), Di Qing Fuxi (wood), Xuan Di Zhuan Xu (water), Yan Di Shennong (fire) and the well-known China ancestor Huangdi Xuanyuan (earth).
In a word, there are different opinions about legends, but they did exist, not just mythical figures. (China's calendar appeared very early, and we attach great importance to it, so basically a lot of information is available in ancient books. )