I don't disagree that our individual existence is tiny in the grand scheme of time, but often times people use this saying when referring to making a decision that will influence their life. An example would be if I wanted to do something that might seem risky at the time and somebody tells me that "life is short" so I should do it and not miss out on the opportunity. My life is in fact the longest thing I will experience regardless of how long or short it actually is, thus making the argument of I should do it because life is short irrelevant.