Why are such long lives recorded in Genesis?

TL;DR:

We aren't certain why people lived such long lives in the time of Genesis, but one possibility is divine intervention: God himself gave people extended lives to accomplish His purposes, such as populating the earth. Those longer lives give us perspective on our own life and death and God's original design for eternal life.

what does the bible say?

The Bible does not provide a direct explanation for why people before the flood lived such long lives. Scholars have proposed many theories, ranging from physical environment to spiritual reasons. Certainly, one effect of those long lives was that people could more quickly populate the earth, as God commanded in Genesis 1:28. A lack of disease and more pristine environment before the flood could have helped extend lives.

Someone who lived for centuries could accumulate vast knowledge, helping create civilization and culture, and making advances in areas such as technology and music (Genesis 4:21-22). Between the times of Adam and Noah, it is reasonable to assume that God’s oral revelation was communicated and preserved from generation to generation until it was ultimately written down in the days of Moses. Extended lifespans helped facilitate the oral tradition. For us today, those long lives contrast with our short lives, highlighting our need to understand physical death and God's plan for eternal life through belief in the person and works of Jesus Christ.

from the old testament

After creating Adam and Eve, God blessed them and said, "Be fruitful and multiply and fill the earth and subdue it" (Genesis 1:28). Genesis 5 presents the genealogical record from Adam to Noah, where Adam lives to be 930 years old; his son Seth, 912 years; Seth’s son, 910 years; and Methuselah, the oldest, 969 years.
After the flood, God blessed Noah and his sons and said to them, "Be fruitful and multiply and fill the earth" (Genesis 9:1). Noah and his sons basically started over to repopulate the earth. Yet scripture shows a progressive shortening of man’s lifespan after the flood. Noah lived 950 years (Genesis 9:29); Noah's son Shem 600 years (Genesis 11:11); Shelah 433 years (Genesis 11:15); Abraham 175 years (Genesis 25:7); and Moses 120 years (Deuteronomy 34:7).
Although Jacob lived to be 130 years old, he lamented that his life had been difficult and was short compared to the lives of his ancestors (Genesis 47:9). By looking back, those long lives in early Genesis testify that death was not part of God's perfect creation. Indeed, God's design was not death, but life, even eternal life.
Psalm 90:10 tells us we are generally given 70 or 80 years to live our life.

from the new testament

Romans 5:12 says that when Adam sinned, sin entered the world. Then Adam's sin brought death, so death spread to everyone, because all sinned. We know from Genesis 1 that death was not intended to be a part of the perfect world before sin entered the world. Thus, death is the penalty for disobedience (Genesis 2:17).
In 2 Timothy 1:10 we see that grace has been manifested through Jesus Christ, who abolished death and brought to light life and immortality through the gospel. That is, the gospel of Christ reverses the curse of mortality, and opens the door of eternal life for all who believe.

implications for today

Our shortened lifespan today is a blessing. A good man could accomplish great things in 900 years, but Genesis 6 illustrates that long life doesn't equate to a good heart. An evil man with 900 years to scheme can do an incredible amount of damage—so much so that God saw fit to destroy the entire world. As God protected fallen man from living forever in Genesis 3:22, He protected us from living long enough to sin as badly as the generations in Genesis had.

As we ponder our own life and inevitable death, we can reflect on the prayer of Moses in Psalm 90. Verse 12 asks God to teach us to number our days in order to realize the brevity of life so we may grow in wisdom. We grow in wisdom by believing the gospel of Christ, who is the wisdom of God (1 Corinthians 1:24;30).

understand

The Bible does not tell us why Genesis records such long lives.
It is possible that God let people live long lives so they could populate the earth.
Sin and the consequences of it likely led to a shortened life span.

reflect

How does reflecting on the long lifespans in Genesis affect your understanding of God's design for life and the impact of sin on mortality?
What does the contrast between long lives in Genesis and the brevity of life today teach you about how you spend your time?
How does the concept of eternal life through Jesus Christ influence your perspective on life and death?

engage

How can we gain wisdom by reflecting on the shorter lifespan we have today, as Psalm 90:12 encourages?
How do the long lives recorded in Genesis illustrate both God's original design and the impact of sin on creation?
How does understanding the Bible's perspective on life and death shape the way we view our own mortality and legacy?