If you don't give your body enough calories to maintain it, it will use fat and cut services to other departments. Different bodies have different ratios for this, so some people will lose a lot of fat, and some will lose very little fat, but suffer from hormonal imbalances, leading to sleep problems, skin problems, mood problems etc. The human body doesn't want to lose fat, it's an energy 'savings account', and nobody wants to deplete their savings.
Carbs encourage the body to keep hold of fat (why would it use up the savings when this ever-so-easy-to-use fuel source is being poured in?) and will keep asking for carbs before it resorts to burning any body fat. That's why we get cravings. If you minimise your carb intake, your body will start to use fat, and once it gets used to doing that, it stops freaking out and yelling 'GIVE ME SOME BREAD/SUGAR' every 3 hours.
So, fundamentally, what's wrong is what we're eating, but not just because it has too many calories; it's also to do with the fact that it provides us with a biological, evolutionary, hormonal drive to eat more of the same, which is almost impossible to resist. Most of us know the 'once you pop, you just can't stop' feeling; the trick is not popping in the first place. The foods that trigger that reaction are the ones with the 'bliss point' ratio of fat to carbs. It's not found anywhere in nature except in human breast milk, so it's addictive (newborns who aren't into it die, so it's evolutionarily programmed) and the big food manufacturers are onto it.
We are a culture of adult bouncing babies.
It takes a different amount of energy for our bodies to process different macronutrients. When we eat 100kcal of carbs, we get about 90kcal to burn/store as fat. When we eat 100kcal of fat, we get about 65% of kcal to burn/store as fat. So, if you eat 2000kcal per day, mostly carbs, that's 1800kcal potentially going on yer bum. If you ate mostly fat, that's only 1300kcal going on yer bum. That's why CICO is hard to calculate, and why people who are 'doing everything right' according to the NHS actually don't really understand what they're going to get out of what they put in.
The NHS 'healthy food plate' is a fat maintenance diet. It's a diet for an already healthy weight person, so that they don't gain or lose any of their already-correct level of fat.