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The long life spans mentioned in the book of Genesis before the flood are often laughed at by unbelievers and even some believers because they simply can't believe people would live nearly 1000 years.  But Jesus had no problem with the book of Genesis.  He even refers to creation and the flood of Noah, as real events.

For believers, who know Jesus and have His Spirit in them, we know God does things beyond our understanding such as creating  a universe and everything in it including human beings, animals and plants forming a variety of ecosystems to make it all work.  We have no issue with God's ability to do things beyond that which we can grasp.  But for those who are ignorant of the reality of God and the supernatural, the strange stories in the Scriptures may seem foolish or superstitious to them like they were to me.  But God can reveal Himself to anyone, especially those who seek the truth.
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So preserving the story of creation and the flood of Noah throughout the generations seems improbable, if not impossible, because it would have been passed through multiple generations.  And without alphabets and writing how could the stories survive?  Well when you look at the lifespans of the men from Adam to Noah, the overlapping years of Adam's life shows that he was alive at the time of Lamech, Noah's father. 
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Adam could have briefed everyone listed in his genealogy, except Noah, on his experience with God and the devil.  Although if he stays consistent with his character in Genesis 3, he would probably still twist the story to deflect blame for the mess he made.  But the creation story could be told from a 1st person perspective up to the time of Lamech.  And he would pass it on to Noah. And it appears Noah lived until the time of Terah, Abraham's father when alphabets were around.  The library at Ebla contained writings from the time of Abraham for example.

So what can we learn from the Scriptures about the lengthy lifespans?  I believe there are interesting details that support the idea of lengthy lifespans in the Bible.  There are also those in the scientific community who study the aging process and death.  Some are puzzled by the design of the human body and why we die at such a young age when our  bodies are designed to last a lot longer than 100 years.  But that's another study.  Let's look at the book of Genesis.   
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Gen 1:6  And God said, “Let there be an expanse in the midst of the waters, and let it separate the waters from the waters.”
Gen 1:7  And God made the expanse and separated the waters that were under the expanse from the waters that were above the expanse. And it was so.
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God initially created what appears to be a totally different environment that what we have today.  "Waters" were above the expanse.  The theory that this refers to a thick moisture layer or a "vapor canopy" in the sky seems plausible to me.  And if this is the case this layer would have kept the earth at somewhat constant, uniformed temperature much like a terrarium allowing plants to abundantly provide food for all life on the planet.  The vapor canopy could have filtered harmful UV rays from the sun preventing the problems associated with excessive UV exposure prolonging life.

It's not until after the flood and the "collapse" of the vapor canopy, during the flood, that we see mention of a rainbow.  Rain, the way we understand it apparently didn't exist before the flood. 
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Gen 1:31  And God saw everything that he had made, and behold, it was very good. And there was evening and there was morning, the sixth day.
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God created man "very good" therefore man's DNA would have been perfect.  No defects, disease, hereditary conditions etc. Other disease would not be present in a "very good" world, at least not for a while..
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But the best support for the long lifespan in my opinion is: 
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  2Ti 3:16  All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, 2Ti 3:17  that the man of God may be complete, equipped for every good work.
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For those who know God and have learned that you can trust Him and His word, the simple answer to the question  "why does the Bible say people lived nearly 1000 years" is easy.  God created them that way.  So I believe it.  And this isn't a "blind faith" rather it's faith built on the foundation of truth I, along with many others, have discovered in Christ and His Word. 
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This of course is ridiculous to unbelievers but anyone who has truly received Christ and believed upon His name is a new creation with the Holy Spirit running the show.  And the Holy Spirit gives a person not only confidence in the Scriptures but also clarity on the power of God.  There is no doubt in my mind that Genesis is true not because I can present a scholarly, scientific defense for it, rather because I know the author.  He has given me life and caused me to see the truth.  
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  1Co 2:14  The natural person does not accept the things of the Spirit of God, for they are folly to him, and he is not able to understand them because they are spiritually discerned.
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Here is an interesting article I came across related to this topic. 


*note - There are different models and dates for theses lifespans but the general idea is the same.