我国的“斤”用了上千年，为何和国际接轨后，恰好是500克？

其他答案已经解释了是人为对齐的。

这里补充另一个点：

世界上绝大多数人类文明，独立演化出的重量单位里都会有一个重量在450-600g左右的单位。

人类学家猜想，因为这接近于一个成年人能正好用手掌抓起的形状圆润完整的石块重量。

When one looks at the weights used in different societies in the past, it is obvious that most units, had they been shaped like a smooth, rounded stone, would have fit comfortably in the palm of a full-grown man. This means that most societies had use for a weight of about 450–600 g. Hence, the oldest unit of weight used by humans were probably weight units of this size. A “mina” weight made of stone from c. 2350 BCE, found in Lagasch, Mesopotamia, that weighed about 477 g, supports this theory. .

——Encyclopaedia of Historical Metrology, Weights, and Measures

因此这样的单位要归并到和国际单位制配合，500g是很自然的。