There are a number of possibilities why people lived longer in Genesis. Although some people want to dismiss the ages as simply exaggerated I believe these are literal ages and not symbolic in any way.
To answer the question, first of all note when the ages dropped off dramatically – right after Noah and the flood. They cut in half for Noah’s sons, then half for their sons, and then finally the next generation – Abraham – lived close to what we’d consider a normal life. This would seem to indicate that the flood caused some kind of change that shortened lifespans.
The first reason that people may have lived longer in Genesis is that God is supernaturally expanding people’s lifespans. God keeps people alive longer so that they populate the earth. If this is case, it is no surprise at all that the person who apparently walks so close to God that God allows him to escape death, Enoch, is the father of the person who lives longer than any other man, Methusaleh. Even among incredible lifespans, Methusaleh is blessed.
The flood caused great changes to the earth. Going back to the second day of creation, we should note what takes place.
And God said, “Let there be an expanse between the waters to separate water from water.” So God made the expanse and separated the water under the expanse from the water above it. And it was so. God called the expanse “sky.” And there was evening, and there was morning—the second day.
We know that the air contains moisture and that clouds are a collection of water in tiny droplets but this appears to be more. At creation, the earth is more like a tropical rain forest that does not need rain because of the canopy of water overhead. Genesis 2:4-6 gives more indication as to what all of this looked like.
This is the account of the heavens and the earth when they were created. When the LORD God made the earth and the heavens- and no shrub of the field had yet appeared on the earth and no plant of the field had yet sprung up, for the LORD God had not sent rain on the earth and there was no man to work the ground, but streams came up from the earth and watered the whole surface of the ground.
There was no rain and the earth essentially took care of itself because of the streams that sprung up and the moisture in the air. The thick atmosphere served as protection from the sun and numerous harmful things like radiation which we know about now. There is no telling what kinds of diseases did not exist because it could not live in such an environment. All of these things would help people live longer.
Another possibility has to do with atmospheric pressure. Scientists believe that the atmospheric pressure was about double what it is now. This makes sense when one considers what all of the extra moisture would do in the air. I don’t understand all of the science behind it but greater pressure seems to help with healing. Hyperbaric chambers are designed specifically to increase the pressure around the body which helps with oxygen in the bloodstream. Football players have started using these in their homes to recover from injuries. The medical community is using them for everything from treating burns, to carbon monoxide poisoning, to healing crush injuries. There are also studies being conducting regarding the use of hyperbaric chambers for the treatment of autism and even certain types of hearing loss. In other words, the world before the flood could have been an environment that allowed the body to heal itself much better than today.
The fourth possibility has to do with genetics. In short, Noah was the last to have the longevity gene. His children had it, but only half because they only got it from their father. The next generation only had one quarter of it. This would give an explanation as to why the ages cut in half from Noah to the next generation.
In truth, we can’t be certain but any or all of these are possibilities as to why people lived longer in the book of Genesis.