(In fact, Noah was commanded to begin construction of the ark a full 120 years in advance (and on top of a mountain) so that people would notice and inquire – and perhaps Noah’s response would stir them to repent in time.) A second possible source is Moses’s lifespan.
Moses lived till exactly 120 (Talmud Sotah 13b) – and on top of it the Torah attests that his energy and vitality did not diminish in the slightest before that time (Deut. We thus bless people today that they be granted the same long, productive life of our great teacher Moses.
In terms of later people who lived slightly longer, the commentators are not especially bothered.
They explain that 120 is a rough limit, not a precise one.
The commentators explain that God’s decree of 120 years was a gradual one.
Man’s lifetime slowly decreased from 900 to 120 by Moses’s time.
Alternatively, life is such a struggle between man’s body and soul, that with so many years to live, a person is bound to succumb over time.
As a result, God decided to shorten man’s lifespan, making 120 years the new limit (Chizkuni, Abarbanel, Malbim, Ha’amek Davar, see also Talmud Chullin 139b and Midrash Ha Gadol).
The Torah thus implied that the perfect lifespan attainable would now be one such as his.) According to the first explanation, that God decreed 120 as the upper limit, why do we find many postdiluvian human beings who exceeded that limit, from the days after the Flood and on?
The average person, knowing he had so many centuries to go, did not fear death and his ultimate encounter with God.
He had many years to live and enjoy himself first, with virtually no sense of his mortality.
"If something extremely bad occurs, people forget all their usual daily worries and become totally preoccupied with this single, truly serious problem.
For example, your worry about your brother's serious illness is pre-eminent and has displaced all other worries, because they all pale in comparison.