1. Insurance is not the same as coverage. Plenty of Americans who do not have health insurance are covered through various programs for children, the elderly, veterans, etc.
2. Insurance is voluntary, and many people who can in fact afford it, simply choose to spend money on "stuff", including that sacred cow of american life, home ownership.
3. The statistics for the uninsured (like your 1-in-6 number) include the elderly, the illegal immigrants, those who are uninsured only part of the year, and other groups that skew the results.
4. If 50MM (although I dispute that number as per above) cannot get the heart surgery because they don't have any coverage but the other 250MM can, it's better than, say, 200MM not being able to get the surgery because of waiting times or ineligibility. Of course, those are totally made up numbers, I am just making a point.