I would also say American's are simply bad at planning for the future, and highlight the fact that 75% of Americans die intestate (or without a Will). But, I think the truth is that is only part of the explanation. While a Will would benefit and be applicable to everyone, not just people with assets, retirement is purely for people who have some type of financial surplus and lets get real. Most Americans are debt financed, from housing, to transportation, to schooling, to even putting food on the table.
It would be very interesting to see how many of those 2/3's who are saving for retirement are actually debt financing their retirement funds (which admittedly makes sense with employer matching), meaning they are living off credit and going (further) into debt while simultaneously saving for retirement. I would venture to guess the numbers are much worse than this article even concludes.