1 - A significant portion of our economy is healthcare middlemen. With single payer or nationalized care, all of our Blue Crosses and Cignas (and the jobs associated) would evaporate overnight.
I'm not arguing that we need trillions of dollars of our GDP locked up in middlemen, but it's very hard for a man to understand something when his salary depends on his not understanding it.
2 - We hear about long wait times in other countries. And the stories are mostly true. There's nothing wrong with waiting, but we've been conditioned to expect near-immediate resolutions.
People in the United States with high-quality private insurance* can usually be seen by a specialist within a few weeks regardless of how severe or immediate their problem is. The truth is that unless that person has some kind of advanced or rapidly progressing disease, they could wait for months without any problems. But that would be a major change to the way we think. It would mean allocating care based on need, not based on ability to pay.
*High-quality private insurance is usually one of the perks of working for a great company. I would argue that only a small minority of Americans have truly high-quality coverage. But all of the people who don't have high-quality coverage envy the others and vote with them, a la "temporarily embarrassed millionaire."