Let’s see if I can add something to this conversation. I’m a fourth year medical student in the United States, who in a few short months will hopefully begin training to be an emergency medicine physician. You are absolutely correct, that the government subsidizes health insurance, and that in a decent number of cases, individuals without insurance or the means to pay for healthcare are eligible for Medicaid. You are also correct that the ideal use of the emergency room is to evaluate for medical emergencies, I say this as someone soon to be an emergency room doctor. Lastly, there are certainly physician groups which are capable of providing cash pay based care.
However, the process to apply for Medicaid can be quite complicated, particularly amongst those with low medical or even just general literacy levels. This disproportionately impacts individuals for whom English is a second language. As I said above, in a perfect world, the emergency department is only for true medical emergencies. However, patients as a whole are notoriously bad at knowing if their symptoms are from an actual emergency or not. Secondarily, in many communities, the emergency department is the only reliable access some individuals have to the health system due to difficult difficulties with transportation and scheduling. With regards to your last point, while there are certainly clinics that can provide cash based care, the majority of individuals who cannot afford insurance are also likely the patient who cannot afford a cash pay clinic.
The fact is also that a large number of uninsured patients will simply have their ER bills written off by the hospital, and/or social workers within the ED will help sign the patients up for Medicaid if they qualify so they become insured can then have the visit billed for, as opposed to the individuals giving fake names.
Unfortunately, the current state of the US Healthcare system is that for many disadvantaged populations, the ER is their primary care physician. This is not ideal, but I will not admonish my patients for doing what they can to seek care in a system that otherwise leaves them abandoned and uncared for