According to the CIA World Factbook, the life expectancy in the United States is currently 79.8, and has the 48th highest figure in the world. Monaco has the highest with a life expectancy almost 10 years higher at 89.5 years.
Death rates have not been static throughout history; for example, the mean life expectancy of Kings of England from 1000 A.D. to 1600 A.D. was only 48 years, so can you imagine what it was like for the general serfs?
If you are planning to retire in your 30s, 40s or even 50s, then the potential future timespan can become very large and stretch into multiples of decades. Financially we have rarely seen such long planning horizons.