Well for many reasons (good and bad) healthcare is more expensive in the US than any other country.
But why is healthcare bought individually more expensive than healthcare an employer buys for all their employees?
One is real and one is hidden.
The hidden price difference is that most employees think they pay $200 per month in health insurance, but actually their employer is paying $600 and chips in $400 so the employee only pays $200. And of course, $200 taken out of a paycheck before you receive it doesn't feel as bad as a monthly $200 charge to your credit card.
The other reason is adverse selection: When an entire employee base gets health insurance together, the people needing expensive care and the people not needing any care all average out together.
But in the individual market, less healthy people are insured but more sick people are. Healthy people forgo health insurance, so the average cost per person rises.