Because their generations are a lot different! Qin Shi Huang was the first emperor in Chinese history, and Liu Bang was the fourth emperor in Chinese history! There were also two emperors between them: Hu Hai of Qin II and Infant of Qin III.
Therefore, although Qin Shi Huang was three years older than Liu Bang, he was three generations older than Liu Bang in terms of generations.
Historically, there have been many illusions caused by this age and generation. For example, the founder of the Sui Dynasty, The Sui Wen Emperor Yang Jian. Yang Jian was originally a Northern Zhou general. After the death of The Northern Zhou Emperor Yuwen Yong, his son Yuwen Yun took the throne. After Yuwen Yun's death, his son Yuwen Yun succeeded to the throne. The following year, Yuwen Xian zen was located in Yangjian, and Northern Zhou perished!
There are also 2 emperors separated between Yang Jian and Yuwen Yong, which makes people think that Yang Jian and Yuwen Yong are not people of the same era, in fact, Yang Jian is two years older than Yuwen Yong.
Yang Jian was born in 541, and the third emperor of Northern Zhou, Yuwen Yong, was born in 543, and Yang Jian was 2 years older than Yuwen Yong.
However, if the two people are of the same generation, it makes people feel that the age gap between them may not be so large, such as Xiang Yu and Liu Bang. In fact, the two of them are the people of the two eras.
Xiang Yu's sub-father Fan Zeng has always been the image of an old master in the film and television drama, it seems that he is much older than Liu Bang, in fact, he is only 10 years older than Liu Bang!