How did people in Biblical times have such long life spans?
God’s power. Another way to phrase that question is “why do we have such short life spans?” I’ve read that scientists still don’t understand why we age and why we die. Why do our cells just start giving up when biologically we should be able to replicate every cell in our bodies until a mutation causes cancer and kills us? Our expected mortality in a sense has just been shifting baseline in which God has ordained it.
Genesis 6:3 tells that that the REASON God began to limit our life span is because of our sinfulness. He wanted to limit the length and height with which each of us could sin so that the wickedness of the flood would not be reached. We’re still wicked of course but can you imagine how much more wicked a cruel dictator could be if he lived 700 years?