So that they can permanently hold that over people's heads as a campaign promise, i.e. "Don't for the Republicans, they're going to take away your free healthcare." Single payer is all about power. The government will have the power to decide what health care you and your family recieve. When they control your health care they control you. At least with a private insurer, if you don't like what they do to you, you can find another company. Not so with the imperial federal government.
One more thing on this, the pre-existing condition thing is a bunch of ******** as well. That would be like me going and buying flood insurance after a damn hurricane that flooded my house. No insurance company would write that policy without someone holding a gun to their heads. Consequently, the government is going to use force to do this to health insurance companies. So someone decides that they don't want health insurance until they get cancer or some other disease that is going to cost a lot of money. The insurance companies are simply going to pass the costs off for this treatment to all the other policy holders, in effect raising the bill on all of the rest of us.
Which is exactly why the "system" (cough) is broken and in need of repair....
Honestly, I think you are thinking with the over-confidence of youth on this issue, Chucktown. Imagine, if you will, being a responsible engineer, paying extra for a really premium health care package for you and your family. Now imagine getting fed up with your job and deciding you want to go back to school to become, oh, something else....
And now imagine that, during this time, you remain a responsible family provider, and you still pay for a reasonably good health insurance package while you are in school. Then, somewhere around age 38, after four years of hard studying, no exercize, too much coffee, too many late night BLTs, you start having chest pains....
And suddenly you find yourself with a "pre-existing condition" that will not be covered by the health insurance offered under your new job... Unless, of course, you go into the military and get covered by their (federal) health care system, which does cover pre-existing conditions...
Or, at the very least, you decide not to go back to school, and to stick it out with your employer, and your employer decides to change health care companies. Suddenly you are required to go to the new insurance company office and fill out all their forms, and lo and behold, you discover that you've been dropped for coverage for future heart problems, or future cancer because you had a mole removed when you were 34, etc. etc. etc. - all pre-existing conditions.
BS? I don't know because I haven't had to have any major procedures done, but I have sat through the interview process with new health care insurers after my employer switched coverage, and been told these ^^^ very things. A neat way for an employer to reduce their heatlh care costs, indeed! Just change insurers mid-stream... Hey, it's the free market, baby.