there are too many things that can happen, an accident on the expressway......the decidedly stupid but still possible 'I forgot passport/computer/coat/etc' at home

I made that GIB flight with exactly 1 minute to spare and the stress I had from not making the connection probably took a full month of my life expectancy

besides in my case since all my flights originate outside the US and I am not American........I rather be totally checked in well within the timeframe that my name needs to be on the list the airline sends to the US goverment