There are 2 primary reasons, both U.S. government based. And no, it's not "more profitable" to keep people fat...but it does cost more to help them to be healthy.
1) in the 1970's the US switched to a corn-based farming system. This was done to combat nationwide dollar-per-calorie price spikes. It was bad enough that is was going to sink Nixon's reelection bid. They found that corn was the best crop to grow in the U.S. due to soil conditions, weather, etc. They paid farmers to switch to corn. It worked too well and prices tanked. The US reps flew to Japan and got introduced to "corn syrup", which could use up the excess corn and could be added to anything to increase calorie counts. (Corn syrup was being used to fatten up cows in Japan, not feed to people) They paid companies to add corn syrup to anything that they could. This worked and lead to calorie dense foods and higher corn utilization.
If the government really wanted to focus on increasing healthy habits, they would need to concentrate at the business level, removing corn syrup (and other cheap-but-unhealthy additives) from food. This would kill the corn-based farming industry. Something similar happened in the 1980's when the corn subsidies were suspended, and it lead to countless farmers going bankrupt and a near collapse of the farm-based rural economy.
2) In the 1980's, the government wanted to cut social costs to increase military spending. Education was a huge cost for the Federal government and became the target of spending cuts. When they looked at the cost per student, the 2 highest costs were teacher salaries and school lunches. They started breaking teacher unions to cut salary costs, but that's a different matter. For school lunches, their hands were tied due to the "food pyramid", which dictated legally that kids needed certain types of nutrition to grow up healthy. They found loopholes in the rules (like classifying ketchup as a vegetable) to cut out nutrition food and replace it with calorie-rich garbage. Since the 1980's, those school lunches have taught kids poor eating habits and gotten them used to that kind of unhealthy food.
If you want to stop obesity, you have to start young. Good food, good habits. That's expensive and would require a pretty big increase in school spending. School spending has mostly migrated from the Department of Education to the individual states. So it would take the Federal government instilling stricts food and food education guidelines that each state would then need to follow. A certain section of the U.S. would consider this "government overreach" and would likely fight these mandates in court. It would also require major spending shifts, which in turn would require some pretty hefty state/county tax increases. Given peoples' aversion to tax increases, anyone who wants to stay in office will (and has) avoid raises taxes for something as "silly" as "feeding kids healthy food".
TL;DR: it would raise food prices, damage the farming industry, increase taxes and likely all be overturned in the court system. When it comes to government, the motto seems to be "Too hard, don't try".