So I get that by cutting out the insurer you are saving some funds, but where does the rest of the difference come from? Are the hospital rooms and surgeons so much cheaper because everyone in those countries pay them rather than just the few people who actually use their services?
* The hospital system in Belgium is 99% either state or non-profit owned. Less sticky fingers. Less luxury.
* Infrastructure (buildings, tools, ...). The infrastructure is extremely heavily state subsidised.
* Medical professional wage negotiations. Medical professionals earn quite a bit less in Belgium than in the US. Quite many (most?) medical pros actually seem to have their priorities right: helping people solve medical problems.
* Choice. In Belgium, people, not insurers choose the hospital they go to, based on reputation and perceived quality.
* Volume. Healthcare in Belgium is fairly accessible, probably one of the most accessible in the world. This means few people putting off operations for financial reasons, so higher volume.
* State subsidies spilling over to foreign patients in unintended ways. I could very well imagine some of the marginal cost of an operation on a non-resident American citizen also being carried by the state in other ways.
Two anecdotes, logical consequences of this system.
1) My (poor by US standards) 90 years old neighbour had a hip replacement at the age of 86, 100% paid by the state.
2) An aunt working as a nurse told me about a phenomenon that was quite common until a few years ago: "winter rabbits", old people "dumped" into hospital for several weeks even if they were not ill. Relatives dumped them at the hospital as a (to them) low cost way of getting rid of them for a few weeks while on holiday for example.