If obesity is a choice then Ancel Keys and his American heart association made that choice with the fat/cholesterol scare which just by sheer coincidence massively benefited the sugar industry and processed food industry.
Eating refined carbs and sugar spikes your blood glucose levels which creates an emergency response in your body by spiking insulin. Insulin tells your body to store fat or burn it (not gonna happen without exercise) which lowers glucose levels back to something normal. Over time your fat cells become insulin resistant, meaning they refuse to store more glucose/fat. The glucose will then have to be stored as fat in organ and muscle (including your heart) tissue. With high insulin your body will simply refuse to burn fat.
Now here comes the big problem. Once your glucose levels go down again (and they do so very quickly) you will feel hungry again. Eating bad food will keep you hungry, ironic, isn't it? It would be, if there weren't profit incentives.
The American heart association used to recommend a high carb low fat diet to prevent heart disease which is ludicrous. People started cutting out fat and loaded up on carbs which are far worse than fat for heart disease.
So ultimately, the problem here is the ability to fill your body with loads of calories while still feeling hungry. Processed food loaded with refined carbs and sugar makes that incredibly easy.
Most grains are bad because people don't mill their own wholegrain flour and then bake their own bread. If everyone did that we wouldn't have an obesity epidemic, we would just have a lot of people that are overweight.
Of course, baking your bread in the 21th century is highly unrealistic for most people. What are they supposed to do then? Well, an alternative would be to just give your body enough time for insulin levels to fall to the point where your body can burn fat. That's fasting and the most popular form is intermittent fasting where you eat during a 8 hour window and then eat nothing for 16 hours. The amusing thing about fasting is that it makes you less hungry, not more, over the long term.
Alternatively, you can do a high protein, high carb diet (1g carb ~ 1g protein) where the protein is supposed to make you feel full.
Ok, finally, you can do a 200 year old diet fad that is known as low carb or the modern variant called keto. If you don't eat carbs your body won't have a glucose spike and with keto your body will have to get all its energy from fat via ketone bodies. Yes, keto lets you eat your eggs and bacon while feeling smug about it.
I have noticed that a lot of people associate high fat with fries, fried food or things like milkshakes. It's kind of amusing because fries are around 30g carbs and 10g fat, a milkshake has 56g carbs and 10g fat. That's low fat, high carb food guys... That milkshake is liquid death by the way.