I was in the metabolic syndrome and obese group, and I have some thyroid problems (likely due to growing up near a uranium enrichment facility). I could diet and exercise and still gain weight that was largely visceral fat, and especially fat in my liver. I was in the beginning stages of non-alcoholic fatty liver disease.

Under my doctor's supervision, I went on the keto diet. I eat high fat foods and meats, and I lost about 50 lbs in 2 months. My weight stabilized over the following months within a healthy range. Additionally, chronic pain, concentration, and energy levels all improved. Most of this is due to the weight loss, but not all of it. Sugar can cause inflammation in high quantities, and it can also cause an increase in blood pressure. Sugars are also metabolized by cancer cells while ketones are not, which would explain the rise in cancer rates as sugar consumption has increased across the planet. My liver function improved, cholesterol levels improved, blood sugar stabilized, and insulin resistance went down.

I personally believe that there is some kind of difference among humans in how we digest things that goes beyond the microbiome. Some people can eat high amounts of carbohydrates and sugars without negative consequences beyond tooth decay. Other people cannot eat any sugar without incurring weight gain and cognitive problems.

Fat by itself is very satiating, you'd struggle to rack up the calories eating purely fat unless you were really trying. However too much sugar without fibre will spike your blood sugar levels which triggers an insulin response, causing your body to effectively store any glucose in your bloodstream into muscle and fat cells[0]. The end result is you're hungry again and able to eat more calories. Combine that with a high fat diet and you have a recipe for obesity.

When I used to weight train, during my cuts (weight loss phases), I had much more success with a high fat/low carb diet than I ever did calorie counting (which naturally makes you avoid fat since they have a higher calorie density).

[0] Not an expert but this is my layman's understanding after reading lots of books on the subject.