It's not that they want us fat, ("they" being Agri-Business and the corrupt collusion with the federal government) they want us to buy products that can be mass produced, and produced cheaply.
Mass produced foods that are also cheap to produce, unfortunately, just happen to be bad for you. Sugar, especially corn sugar, is extremely cheap. Therefore the American diet is higher in sugar than any other modern civilization on earth. Including the French.
Sodium is far-far cheaper than herbs and spices. Therefore mass produced foods are high in sodium. Therefore Americans ingest boatloads of sodium in our diet.
Foods that are good for us, are difficult and/or expensive to mass produce. Therefore the government, who have FAR more to gain financially from the massive agri-business lobby and the sugar industry than the local farmer who gives them nothing - deeply subsidize both to make the products cheaper than local made products.
And Americans like cheap more than anything else - even if it is killing them.