Alright, now imagine the government
doesn't pay for healthcare. This is the literal cheapest thing for them, as it now costs $0,
and they don't have to spend time worrying about law.
If the government isn't paying for your healthcare, then you have to pay it yourself. If you are fat, statistically, you will probably end up paying a lot more over time. This is not "wasted" money - no money you spend on your health is totally wasted. The good news is, you personally benefit from this money more than anyone else who could possibly spend it on your behalf - you reap the health benefits, the extra Christmases with the family, the bigger smiles from strangers.
Now how much does being fat cost over a lifetime? Frequently in the range of tens or even hundreds of thousands of dollars. That's pure medical costs, before we even consider how much the intangibles are worth.
The thing is: Most people understand this. If you understand this, and are still fat, then on some level you are indeed saying that the money isn't worth the trade-off of changing your whole lifestyle to (a) fix this and then (b) ensure you don't end up there again.
To me that's proof that being fat is a very hard thing to change just by throwing money at it. The literal best, most incentivized person, the fat person themselves, can't even figure out how to change it, not even with at minimum tens of thousands of dollars on the line. There are probably government interventions that probably could move the needle at scale, but they are probably not "let's subsidize spinach farmers" or whatever, because an extra $2 per week for spinach is just not that much money compared to even a single $50,000 gastric bypass. They probably look more like "If your BMI drifts above 27 we send you to weight loss prison and you can come out when you're below 22 again", which sounds insane, because it is. It is simply a very hard problem to solve by throwing money at it.