Why did the "rulers" of ancient China not implement the current land ownership system: public ownership of land and family labor? Wouldn’t this prevent land annexation?

The period of land equalization existed. During the Northern Dynasties, in order to improve the level of the national economy in the short term, the land equalization system implemented by the Northern Wei Dynasty and other dynasties had the effect you mentioned in a sense, which was to mobilize the production enthusiasm of farmers under the small-scale peasant economic system through the state's distribution of land. Prevent land annexation through regulations.

However, in the middle and early Tang Dynasty, especially during the Wu Dynasty, due to the large-scale annexation of land, the equal land system was destroyed.

There are many reasons for the destruction, such as the unavoidable emergence of private ownership of land by large landowners in China's official system; there is also the fact that the equalization of land system was established during the militarism period, and the Northern Dynasties were aggressive in military affairs. Farming during the farming season and training in the free time were the easiest to implement under the individualized land equalization system. However, by the time of peace in the early mid-Tang Dynasty, the equalization of land system could no longer adapt to the economic development under the country's high taxation conditions, so it naturally declined.