On what basis do you accept your scripture as truth?
Vedic knowledge presents itself as factual, and we've found no reason to doubt it. Those familiar with their comprehensive, consistent, and detailed information on so many aspects of human endeavor—spiritual, ethical, and practical—would likely find it hard to believe that they could have been fabricated with no basis in fact.
Respected scholars—past and present, east and west, secular and traditional—accept the knowledge contained in the Vedas—including the Srimad-Bhagavatam and Bhagavad-gita—as both factual and timeless. Vedic teachings are the basis of one of the world's most enduring cultures. We wouldn't be interested in them if we thought they were fiction. Their scope and accuracy—along with considerable reliable testimony attesting to their validity—make it difficult for us to dismiss them as make-believe.
Fiction can't help us. If we mention a health concern to our doctor, for example, we wouldn't expect him to base his diagnosis and treatment on something he read in a book of fairy tales. We would hope he has factual knowledge of how the human body works, and experience dealing with a wide variety of diseases. Likewise, if we're looking for spiritual knowledge, knowledge of the self beyond the body, we don't want to waste our time—and life—with information that may or may not be true.