Why didn't ancient carpenters in China use nails?

Because nails had not been made at that time.

The craftsmen in ancient China were very clever, and they used the most environmentally-friendly and ingenious methods now, but there was a tenon structure, that is, the protruding part of the component was called tenon, and the hole where the tenon was installed in the wood was called mortise. This mortise-and-tenon structure looks "loose", but if the steel bars and nails are distorted by external forces, they cannot be restored, but are mortise-and-tenon structures. Of course, this skill has continued until now, and we can still see it on many wooden furniture, but the appearance of nails has made this skill rigid. Of course, nailing has also brought great convenience, in line with the current attitude of quick success and instant benefit.