Data is no ordinary human, but the merged energies of Lord Brahma, Lord Vishnu and Lord Shiva. The four dogs represent the four Vedas, and his appearance as an ascetic shows his perfect renunciation. the cow is the spirit of the earth, seeking His protection. So Lord Dattatreya is the perfect renunciate. And He is said to be alive to this day, alive and elusive.
And this perhaps brings us the heart of another question. What is the Guru? Does a Guru teach? Or is the Guru's presence merely the teaching? A Guru is one whose separate individuality has been merged by the expansion of consciousness, by enlightenment... and from a different angle, a guru is one who has perfectly bowed down, and is just a container for the awakened Divine, the union of Shiva and Shakti.
Does Lord Dattatreya teach, then? Yes. He points us, especially in the famous passage from Srimad Bhagavatam, from the lessons available to us by deep connections with actual lived reality, and his is the voice thatr details 24 gurus taken from the world around us.