This is what's known as an 'anecdote', and is at the crux of the issues surrounding men's weight loss. This approach, and other very obvious and straightforward ones do indeed work perfectly well for a fair fraction of the population, but they perform poorly for another large fraction, and
fail abysmally for yet a third.
While there are no doubt plenty of people that could lose weight fairly readily if they gave it some basic effort, most of the people who are actively trying to do so (enough to pay for a service that supports that, for example) are in the second and third categories. And yet every time anyone posts in a non-dedicated forum anything about systems and plans to help that group solve their problems, there are a dozen people like you popping in to tell us "it's not that hard, just give it a try!"
I regularly make 60 mile bike rides, I can jog 10 miles (though I can't walk the next day when I do), I play tennis about 7 hours a week. I weigh 465 pounds. Trust me, I've tried the approach you're describing here. I once managed to hold onto it for 8 months, during which I lost about thirty pounds and then plateaued, while constantly fighting my body. I was sick twice as often as usual, and I had to carefully micromanage my calorie intake to make sure I had available calories for any physical activity I wanted to engage in, all while constantly battling cravings for dozens of foods _I don't even like_.
It would be a simpler world if all of us fat people were just fat because we're lazy, but the truth is that people vary really a lot, physically and metabolically. Your experience of the world is valid, but not universal.