You're right that obesity causes low testosterone, but low testosterone also causes obesity (plus factors that lead to obesity, like lethargy). Chicken/egg problem?
Am I missing a group of people who eat exactly the kcal they require, get the nutrients they need and don't have a metabolic problem, but still materialise fat for some reason?
Breaking those circles is not easy at all. It's closer to rehabilitation and a long process.
Of course reducing intake is a main goal, but then keeping it low is as important.
There's too much evidence that our obesity problem is caused by excess caloric intake.
One factor is processed foods. They often either have lower satiety/calorie ratios, and refined sugars with no accompanying fiber has actually been shown to decrease satiety and make a person hungrier.
Another factor is sedentary lifestyle. Peoples' appetites tend to be somewhat fixed, and if you used to burn 2100 calories a day on a 2100 calorie diet, but now you burn 2000 calories, that adds up. It doesn't seem like much, but that's 1 pound every 35 days, or 10.42 pounds every year.
Another factor is just evolutionary. In human history, broadly, it makes sense to eat at a surplus whenever possible, because except for very very recently, food surpluses were rare occurrences, and having that extra fat storage on your body could mean the difference between starving and living through a famine.