Mainly because of the absolute amazing levels of inefficiency our government shows in other services.
Maybe this is just a difference in American culture. I'm not being snarky or sly here, I mean it. Do other countries have some kind of uber-efficient post offices and DMV equivilents that make them more willing to trust their governments to get them a new kidney on time?
But while I'm not outright opposed to socialized health care... yeah I will admit when the topic comes I think of all the hours I've spend quagmired in the beauracracy of our other government provided services and have a sort of cold chill go down my spine. A hospital run like the DMV would be the closest thing I can imagine to pure Hell.
A serious question: Why is it so common for people to make the point of comparison for government run institutions the DMV, and not, for instance, NIST, the CDC or the IRS (which all seem to run reasonably competently, even if the tax code in the US makes actually doing taxes an apparent nightmare)?
The DMV isn't even a federal organization, so far as I'm aware.