There goes that simplicity then. How do you pick one? How do you stay on it? How do you decide whether to switch, when to switch and what to switch to? How do you motivate yourself to try again once you've failed? And on it goes.
How do you pick any activity that encourages you to burn more calories than usual?
I would expect that comes down to finding something you enjoy doing.
Exercise and eating well is a way of life. If your life dictates you do neither, you will get fat. I don't really understand how its complicated. You just do it for the benefits.
Personally I eat chicken and veg 5 days a week (with occasional carbs when I train), train for 3 days and climb once a week. I also mostly hit my 10k steps per day.
They have no willpower. You are the only person who controls what you eat everyday. If you are in stuck in an office you need to block out time for exercise.
How can there be any other excuse?
I know people who look great and eat entire cakes each day but they do 4 hours of high intensity exercise everyday. I can't do that as I am fundamentally lazy and only exercise/diet for vanity.
IMO you have an oversimplified view of this based on limited experience. You have abstracted away the diverse and complex reasons this doesn't work for everyone behind the excuse that people make excuses. It's not so simple. People are different; their circumstances are different; we can't just write them off by blaming them for being obese or addicted.