1/3 are illegal immigrants. Those folks would be excluded from the universal healthcare system of nearly every EU country. France and Spain are the exceptions, but France is under a lot of pressure to scrap that.
1/3 are eligible for Medicaid but haven’t signed up.
1/3 can afford Obamacare premiums but choose not to buy insurance. The Obamacare premiums are capped at 9.5% of income. That’s about the same as the employee-side healthcare tax would be in France or other countries with universal healthcare.