The simple answer is “because we don’t listen very well.”
Whether it is the teachings of Jesus, or the stories of our forebears in faith, the Bible does seem to be a little bit redundant at points. Many of the most important tales and precepts are repeated verbatim at multiple points in the text, while others can be seen to reecho with only minor variation. Still other sections are so similar one-to-another that it is easy to think that they could, in fact, be the same story re-told over and over again.
At times, the Holy Writ seems to be in want of a good editor. I would argue, that editing is precisely the point.
For a book compiled from different sources spanning thousands of years and worlds of culture, the Bible is remarkably concise. Moreover, the ability that its multiple editors have to bring together disparate stories from disparate peoples into some kind of master narrative of faith is nothing short of remarkable, in-spite of the repeats. The very notion that the same story might have been important to multiple groups of people is a sign of the very breadth of God’s love toward all of the people of the world. Moreover, the sometimes subtle differences between exceedingly similar stories reflect the equally subtle differences between diverse people and cultures that have found a way to share the same faith. Far from being redundant – even though redundancy might be the best thing for us sometimes – the Bible is intentionally inclusive in its own peculiar way.