Why are valuable calligraphy works called soft gold?

Good calligraphy may be 3000 square meters, or even 10000, tens of thousands of square meters. They are the true representatives of China traditional culture! They have been circulating for thousands of years and have great influence all over the world. They are national treasures and wonders of the world! ! ! And you can't copy it! ! ! Most of what remains now is modern, rarely ancient, and less and less because of preservation and various reasons! ! ! Among them, good calligraphy will surpass soft gold! ! !