(Ok, ok, unless kept firmly in check by anti-Reaganesque onerous regulations, and even then I have my doubts.)
BECAUSE IT COSTS MONEY TO TAKE CARE OF SICK PEOPLE! (Ahem, please forgive the caps.)
If you have a preexisting condition, you simply cannot get insurance to cover it. This is not a difficult point. Why should a rational for-profit entity give you coverage? This is why it makes no sense (to me) to have our health care rationed by for-profit institutions. You get sick, they get busy finding a way to deny you coverage. The worse they treat you, the more money they make. Yahoo! Martinis and stock options all around.
How many people are uninsured? In 2007, it was 27% of the population under 65. (Because we already have socialized medicine for the 65 and older set.) On edit: New numbers from 2009 paint an even darker picture. 45,000 unnecessary deaths per year; that works out to a 40% greater risk of death if uninsured. Wow.
What happens when you don't have health care? 13,000 people between 55 and 64 died in 2002 because they didn't have health insurance. (As an aside, the study that gives the 13,000 number is great. It looks at about every confounding factor you can imagine). Just since 2002 (without adjusting for anything), that's a death toll equivalent to 24.5 World Trade Center bombings (for only a slice of Americans). I seem to recall that we've racked up $1.6 Trillion and over 5,000 American lives to pay for the War on Terror. How much is health care supposed to cost? About a hundred million a year. Less than the War on Terror.
What's our problem? I guess it's the Quentin Tarantino effect--no chases, plane wrecks, explosions to show on TV, so it doesn't register in the popular imagination. People without health care die quietly off-camera, but they still die.