Upvote:1
The answer is simple; it has nothing to do with divinity, even less to do with lifestyle, and everything to do with genetics.
We know from modern genetics that inbreeding tends to result in... problems.
We know from Scripture that the human population was at one point reduced to eight individuals. In fact, you (almost) answered your own question: "Noah was the only pure blood". This isn't strictly true; all eight people on the ark were "pure blood"... but their descendants weren't, and Scripture records life expectancy taking a nosedive in the next generations before stabilizing again at about what it is today.
Obviously, we don't know exactly what genes were lost or altered, but by far the most reasonable explanation is that the genetic bottleneck caused by the Flood resulted in some manner of genetic degradation, which in turn lead to the much shorter life expectancy we have today. This is also consistent with the effect taking several generations to be fully realized.