Why are there so few original calligraphy works in Tang and Song Dynasties?

Because of the long time interval. The older things are, the rarer they are. Like the two kings of the Eastern Jin Dynasty-Wang Xizhi and Wang Xianzhi-don't there exist any original works? The only authentic calligraphy that survived in the Southern and Northern Dynasties was Wei Bei. And paper breaks easily. Even if our ordinary paper is left for decades, it will be oxidized to be fragile. Even good paper can be preserved for thousands of years. How much? Coupled with mobilization and war, it is not surprising that the original calligraphy can't stay.