You grew up eating healthy food. If left to your own devices you probably maintain a healthy diet, regular exercise, and stay at a healthy weight. Whether it's a condition of genetics, environment, or both you're privileged to have an easy time maintaining your weight; it's not that simple for many people. Especially in first world countries where unhealthy food is cheap, fast, widely available, and tastes better. Cooking veggies and rice requires a trip to the grocery store and cooking time and doesn't taste nearly as good as that Big Mac.

I lost about 70 pounds (~32kg) in 2 years through only diet. At the start when I just catalogued what I was eating I was eating about 4k calories a day. I ate more than the people around me, but not a huge amount and I rarely had stomach problems or felt "stuffed". That's just how much food my body wants to eat in a day. When I tried to eat less than that I would feel hungry. Religiously recording what I ate and planning ahead I managed to lose a lot of weight by staying to ~2k calories a day.

But if I stop recording food everyday? Within 2 months I'm at about 3k calories a day. At ~2k calories per day I spend most of my day feeling hungry. I have tried 1-2 large meals a day, or 5-6 small "snack" sized meals a day with no change. My natural preference is to eat a lot of high-fat and high-carb food that is calorically dense and if I don't actively work to curb that tendency then I start putting on weight. For me to be healthy requires constant, daily, effort on my part. It will never be easy. I can never relax on my diet.

Everyone is different, but it is not that we don't care or that we don't try, it's that it takes a lot of concerted effort to be healthy for some people. Like anything that requires willpower (quitting cigarettes, cutting back on booze or weed, quitting video games or porn or really any unhealthy mental addiction, going to work everyday, staying in a committed relationship, etc.) for some people it's trivial and for other's it requires concentration, motivation, and hard work and despite all of that we still fail much of the time. Controlling your weight means controlling those urges forever for some people.