The US isn’t the only place people eat like shit, though.
The french and Aussies and English and Germans and Italians all have some unhealthy food habits, or excessive butter/cheese, or elevated alcohol consumption levels, etc.
You can point to activity levels, but weight is about food consumption, not activity levels, and (eg) Aussies are a pretty car-reliant culture too. Pretty much any way you can argue that the US is "unique" - it's not, we are a culture exporter and some other place has inevitably picked up some of our habits.
Americans aren't even uniquely fat anymore. There are other populations (like aussies) that are getting close to our obesity levels and they still don't 5x their healthcare spending. And the idea that obesity makes up even a simple majority of total healthcare costs (as it would need to be for this to be the factor that 2-5x's healthcare spending) is generally ridiculous.
(also, paradoxically, a lot of things that we think of as unhealthy actually reduce total life healthcare spending, because US care is heavily weighted towards end-of-life care. It's cheaper to die of lung cancer at 40 than to have three knee replacements and spend a month in the ICU at 80, and US health care is heavily weighted towards the latter because of medicare, everyone else struggles to get affordable access to the system.)
Like it would be interesting to see an attribution of what specifically americans are eating so differently vs everyone else, and the caloric/macronutrient balance and total inflow/outflow, and total life expectancy (and total spending) changes. I suspect it's just a "how much of it" more than anything, but the "gosh americans just eat like shit, that's why!" really doesn't justify 5x per-capita expenditures on the face of it, in the way people try to use it. The rest of the world drives cars and eats like shit, and eats too much too, in varying proportions.
But to go back to spending - it's really hard to argue that it's any one thing. It's everything. Everyone at every layer is bleeding everyone else. Americans are a bit unhealthier, but we also have for-profit hospitals (even non-profits make a "profit", it just goes into real estate investments), for-profit insurance, doctors who make $400k because they need to pay off $300k of loans from for-profit education, drug and device manufacturers who have no cost containment before approvals, a political system that tolerates evergreening and similar practices, an "access problem" for delivering basic care before it escalates into expensive problems (even "preventative care" often still ends up costing), etc etc.
That is really the problem and why the system cannot be reformed. Medical care is at the nexus of every single "unsolvable problem" of the US political system, from patents to education to farm-lobby and environmental regulation, and it involves a million taking hands that after a true reform will no longer be able to take. There is a lot of people who have become reliant on the revenue streams from this rent-seeking/unproductive activity, and they will lobby heavily to make sure they can keep taking.