When should an author disclaim historical knowledge?
Without some statement regarding the early life of the subject of the biography, it is impossible to tell whether the author looked hard and found nothing or just didn't bother.

By writing that little is known, it indicates that the author (or others whose work the biography draws on) has made efforts towards finding such information but that it is not available.

Pointing out the explicit difference helps readers who either wish to learn more or would wonder (as you seem to have done) at the lack of detail.