Spoken like someone who has never struggled with weight? If so, then enjoy your good health (I mean that sincerely)!
As for me? My body's relationship with food is broken. What a person who doesn't struggle with weight doesn't understand is that it's not a matter of calories in, calories out. Food is information. And for some of us, either due to genetics or bad choices (in the past or currently), the information that our bodies get from food makes us hold on to fat and toxins. You and I could eat the same things and have the same level of activity over a period of time and chances are, I would gain weight on what you eat and how you move. Why? Because how my body processes food isn't the same as how yours does. Maybe my genetics are such that my gut doesn't handle certain things as well as yours does, so I get low-level inflammation from dairy or certain grains, inflammation that triggers an insulin response in my body that triggers a excess fat storage. Or maybe you were breastfed longer than I was so your digestive tract wasn't subtly re-programmed to not receive micronutrients like mine perhaps was from the synthetic nutrition of formula. Maybe you've never had a diet soda habit, which truly messes up with your body's insulin response and fat storage in ways that certainly weren't understood 20 years ago and even still aren't understood by many people today.
At the end of the day, I can tell you that after 20 years of repeated concentrated efforts to just eat less, eat healthy and exercise more, IT DOESN'T WORK. "Everything in moderation," "calories in, calories out," "just eat less and exercise more" is literally killing us. My body responds in a toxic way to many foods now. Now that I've eliminated grains, especially gluten, if I do have grains, I have a noticeable response - joint and muscle pain, bloating and constipation. If I have dairy, I have stomach pain for 24-36 hours until it's out of my system. I have a lot of weight to lose, so although my fasting blood glucose and post-prandial glucose numbers are considered health by conventional medicine, I'm eliminating all added sugars and most fruits and am keeping my fructose intake to under 15 grams a day so that I keep my insulin levels low and in fat burning mode instead of fat storage mode.
I'm one of those people who has been healthy but overweight for many years, and each year it's another five lbs, and another, and another. Cholesterol, triglycerides, glucose and insulin are all great by conventional medicine's standards. My diet has been mostly whole, fresh foods with very little by way of packaged foods (things like BBQ sauce, salad dressing, wheat thins, etc.) for many years. I run several 5K races a year, do one triathlon a year, take zumba classes, yoga, lift weights, biked a 150 mile event last year, etc. So I eat right and exercise...and am 75 lbs overweight.
When you get to the point where something is obviously broken, you fix it. For me, the fix appears to be that I simply need to eliminate whole categories of foods from my diet, add in more of the right kinds of foods (more fat, actually - avacado, coconut oil, nuts), and use some supplemental items to help speed the detox and reset along. I hope to get to a point where I restore some balance to my body and can add back in some things I used to enjoy, but I'm at peace with the fact that I may never get there and that's OK as long as I'm healthy and feeling well.
Sorry to have written an essay on this but it's one of those things that people really don't understand unless it's been a problem for them.
FWIW, both of my parents are slender, active and healthy. My mom has some chronic health issues such as fibromyalgia and chronic fatigue - both possibly related to untreated chronic lyme disease - but she manages her symptoms with diet and supplements. I had a very, very healthy diet growing up - organic foods, home-grown produce from the garden, real meals, whole grains, and no sugar or artificial colors or sweeteners. I think was really started by downward spiral was that I worked in fast-food restaurants as a teenager and in college so I ate that horrible food, drank diet soda like water, haven't slept more than 5-6 hours a night on a regular basis since middle school, and would gain weight after each pregnancy was over (would lose weight during the pregnancy). So clearly I made poor choices as a teenager and young adult that I'm paying for and correcting now!