Why do you think those before the flood had such a long life, then after the flood the age grew shorter and shorter?

Because we are becoming more and moe imperfect, the further in time we are from Adam and Eve...who were created perfect, then became imperfect because they had disobeyed...It was they who brought death on mankind.

Being perfect, they would never have died...[ eventually]

(Genesis 2:16, 17) Jehovah God also gave this command to the man: “From every tree of the garden you may eat to satisfaction. 17 But as for the tree of the knowledge of good and bad, you must not eat from it, for in the day you eat from it you will certainly die.”

Earth before the flood had to be life sustaining, after Adam and sin, there's no life to sustain, a long life did exist until flood year 1656, evil was gone with the flood, so was life sustaining earth, the more time passing after Adam and the flood, the shorter life became. 70 or 80, at old age, Psalms 90:10, is nothing compared to age 962 or 969.

Because God decreed it. His method was that the cloud cover surrounding our planet kept the radiation from the sun away from us. Then, because the clouds were used to flood the earth, they no longer served to protect us. We actually die from radiation poisoning.

Natural genetic degradation the Bible lists the chronology so you can see how the lifespan decreased slowly until god said that the natural life span would become 70 or 80 .

The longer life was to aid in populating the whole earth.