There are bright moon, clear spring, huannv, fishing boat and so on in the poems. Why does the poet still say that this mountain is "empty"?

First of all, it lays a tone for the poem: ethereal, and then it is combined with the actions of "Huan Nv" and "Fishing Boat" to express the poet's yearning for this kind of life.

You can combine the whole poem. Huannv and fishing boats are obviously related to water. They move on or near the water surface and have little to do with the state of the mountain. When Huannu came back, she crossed the bamboo forest and began to be related to mountains, but it was later on the time axis than "Empty Mountain and New Rain". The difference in time can also be seen from the word "Hou". Therefore, there is no contradiction between "Huannv", "Fishing Boat" and "Empty Mountain".