Why is Japan not an "ancient civilization"?

The earliest surviving cultural relic in Japanese history, the so-called important cultural treasure, is Takamatsu Mausoleum, which is equivalent to the Tang Dynasty in China. Although the Japanese government has organized cultural heritage management and protection units to carry out a series of archaeological excavations since the last century, it turns out that Japan really can't help the ranks of ancient civilizations. In the composition of Japanese culture, China ancient culture accounts for a large proportion.