I think it's because the US is a deeply unequal country. Services are good if you can afford them, and it just so happens that more and more americans can't.

The current 44% of U.S. adults who say the quality of healthcare is excellent (11%) or good (33%) is down by a total of 10 percentage points since 2020 after steadily eroding each year. Between 2001 and 2020, majorities ranging from 52% to 62% rated U.S. healthcare quality positively; now, 54% say it is only fair (38%) or poor (16%).

Only four years of inflation and positive opinion dropped 10 points. That's massive.

They are also asking about healthcare overall. So people aren't rating health insurers directly, but their personal doctors and hospitals. Flip the question slightly and the numbers change:

Americans' largely negative views regarding healthcare coverage and quality in the U.S. likely contribute to the widespread perception that the overall healthcare system has major problems (54%) or is in a state of crisis (16%). The seven in 10 Americans now holding these views is in line with the trend average of 69%.

Twenty-five percent of U.S. adults say the system has minor problems, and just 3% think it has no problems.

I'll bet that if you ask people what they think of their insurer, they'll have opinions.

Another comment on a different post said that these polls don’t differentiate between private and Medicare/medicaid plans and they also have a bias where respondents don’t need to have used their insurance benefits (I’m guessing beyond regular preventive care)

If you are "healthy" and just need to go to your regular preventive appointments, having read nothing from other peoples experiences, i could see rating it as "fair"