Why only sweep graves for three generations?

The first generation is parents, the second generation is grandparents, and the third generation is great-grandparents. There are three main reasons why there is a saying that there are no more than three generations of grave sweeping: the reason why you can't sweep is understandable. If every generation needs to sweep graves, I don't know that there are dozens, hundreds or even thousands of graves since ancient times. Where can we clean them? There is an old saying that "one generation of relatives, the second generation of representatives, three generations and four generations are inseparable", but people's life span is only a few decades, most of them run in the it family, and it is not easy for four generations to live together. For many people.

Grave-sweeping is a kind of "grave sacrifice" and "respect for time thinking" to ancestors, and its custom has a long history. In the north, it is called going to the grave; in some places, it is called "sweeping mountains" or "worshipping mountains"; in the south, it is called "hanging mountains" or "hanging paper".