I've recently booked a holiday and one of the terms was travel insurance. They just took my word for it but I'd never travel without insurance. When your travel insurance covers you for 10 million then that's why you need to take it out.
ANY sort of illness could finish up costing you thousands of pounds or even hundreds of thousands of pounds.
I did a google search and found a story of one lady who got a major illness in the USA and because she had not declared a previous illness the insurance company would not pay out.
She was stuck in the USA for 6 weeks and faced a medical bill for 200,000 dollars.
So it would be madness to go to the USA on holiday (or anywhere abroad on holiday to be fair) without travel insurance.
yes, as mentioned, medical care in the USA can be stupendously expensive. Not so bad in other countries. You can take the risk and hope you don't fall ill; but if you do fall ill, you could end up selling the house and putting the kids to work cleaning chimneys.
My friend got knocked over in America by an ambulance. Not his fault but they wouldn't pay compensation until he settled his medical bills. Crazy system.
If your question means "Will I be barred from entry to the USA if I've not got travel insurance?" the answer is "No".
However, as others have indicated, you'd be mad to visit the USA without it. A simple trip accident, involving an ambulance journey to a hospital and (after you've been found to have no serious injuries) a plaster cast being put on your wrist could leave you with a bill for many thousands of dollars.