If the average life expectancy is close to 75yrs...?

If the average life expectancy is close to 75yrs for men in the US why are we told to save 20% or more of your annually income? just look in your local obituaries that should tell you to just live day to day. why stress and save when you have more then a 50% chance of never spending all your money. I must say i rather live pay check to pay check and enjoy my spending money each week ($400.00 extra cash).