One reason it costs more: people with serious conditions may delay presenting because they fear the cost of healthcare.
And unfortunately, many serious conditions are dramatically more expensive, and potentially more lethal, when presenting late. It's a nasty piece of positive cost feedback inherent to the current US system, and one that disproportionately affects patients from lower income brackets.
Other reasons include the massive costs of running and staffing the billing infrastructure, cost of compliance & provider insurance, and uncontrolled drug pricing.
These factors are not present in most other first-world healthcare systems, or are present but minimised by effective regulation.
