The Vedas and Upanishads were not the products of logical thought and reasoning. Those who composed them were rare spiritually gifted and advanced individuals who, during meditation, were able to vibrate in tune with the Universe (the Universal Intelligence). Once they were in that frequency, the individual simply merged or vanished. I, me and mine ceased to exist and the truth which emerged into words and language came to be recorded in the Vedas and Upanishads.
Those ancient rishis or sages understood clearly that whey they experienced and received was not theirs. The "I" disappeared and they felt no need nor desire to claim authorship. offering the insights freely and anonymously for the benefit of humanity.