Because if only he, and a few others did it, it would not be significant enough. He realizes he makes an enormous amount of money of other people's work. As do most all billionaires.
He is arguing the principal that someone of the working class, i.e. his secretary, should not have to pay a greater percentage of their income than him. His income is well over what he needs to house himself, get good food, medical care, etc.
And he is right. Some super wealthy people are nearly like Pharaohs of Egypt. And some people do treat them like gods because their are rich/and or famous.
Simple example: because of his industries he uses a greater percentage of the USA services - such as roads for transporting goods. So therefore, he should pay a higher percentage of maintenance of said roads.
And nobody is talking about returning the tax rate on the wealthy to 98%; that probably was a bit excessive.