It often happens with insured people showing up at the ER as well. Many patients don't pay their bills, either because they can't afford it or are just deadbeats. As overall healthcare costs have increased, insurers and employers have shifted more costs to plan members through higher deductibles / co-pays / co-insurance. So hospitals end up with a lot more bad debt, and this in turn causes further price inflation.
Carriers and providers don't make this easy, either.
I had a recent doctor's visit, with very good healthcare coverage, that was an in-network facility but an out-of-network provider. Coverage would have kicked in if I had a referral from my PCP, but my PCP recently retired and I was advocating for my own health for a small dermatological issue. They said it was cosmetic, my old PCP said it was not. I got a stack of 10 bills over many months all stating different things -- everything from $0 EOB to over $2k in uncovered expenses. No one would take ownership of sorting out what I was on the hook for. No one I talked to was empowered to actually solve it. It wasn't an affordability problem over $2k (but would be for my elderly grandparents on fixed income). Even asking "If I give you $2k does that resolve the debt?" was answered with "we won't know until we apply the payment" type non-answers.
Burn the whole stack down -or- earn enough you can operate on cash for the tier of care you want. Nothing inbetween seems to be working.