The reason it’s hard to build muscle, is muscles are incredibly inefficient and a huge calorie sink.

If you’re heavily muscled, you’re using more calories even if you’re not working out. Just reaching out to grab something takes more energy.

So building muscle means you have a higher caloric baseline.

And that’s not even getting into calorie density. 150 calories worth of beans will keep you feeling full for a long time, and a 12 ounce can of soda won’t make anyone feel full because it’s all liquid already.

Then there’s physically eating slower and chewing more, because we evolved to not feel full if we’re still eating.

Reducing it just down to “eat less calories than you use” is technically correct, but it’s the details that help people.