Why should Chinese painting be left blank?

There were many blank places in ancient times. Not only calligraphy and painting will be left blank, but poetry will also be left blank. The rest is the further and deeper expression level of painting or poetry. Traditional Chinese painting generally leaves a blank space in the upper right corner, mostly for inscription and seal, and some are purely for deepening the artistic conception of the painting. Moreover, if Chinese painting is poured like a duck, it will have no lasting appeal.

I'm not sure what you mean by blank space. Usually, when Chinese painting is mounted, the paper is in the middle, with a white edge of about one inch around it, which is for beauty.