If you are actually poor in the US, you qualify for free or highly subsidized healthcare. US has plenty of those social welfare programs, and people online typically are well off enough that they aren’t even aware of them.

The question was why people don't like universal health care and not an objective measurement of its value to society as a while. Around 10% of the US population is uninsured so 90% does have insurance. So 90% of the population isn't comparing it to a system with no-insurance.

Also worth adding is that most of those uninsured are present in the country illegally. For comparison, UK’s NHS does not offer free services beyond primary care to those unlawfully present either, and I think this is true of most other public healthcare systems around the world.

But it is ethical to keep the lines shorter by excluding the poor?

No but that wasn't the question the OP asked. Unless you've got a nation of saints the majority of people will make somewhat selfish decisions.

And for most citizens of developed countries that selfishness translates to universal health care. No need to worry.