science

It's both a behavioural issue and a complex bodily disorder with many external factors...

Biologically, weight is pretty much an effect of calories in and calories out. If you lock someone up and give them too little food, they'll generally get thin. The body can't create fat if you don't feed it and it can't work without burning energy. Physics.

But losing weight when you're not locked in a cell with someone else controlling your food availability is really hard. Not eating when you're hungry is hard. The facility of getting healthy food that makes it easier is socio-economical. etc

It's like running a marathon is "just" about starting to run and not stopping until you reach the finish line. It's trivial on one level, really hard on another. It's simple physics AND a complex web of genetic factors, motivation, knowledge, upbringing, etc

So most people are technically and biologically capable of losing weight, but most people are also practically and statistically not very successful at it.

Most popular diets work under controlled conditions, for the people who adhere to them; but most popular diets also don't work in practice, as it's too hard for people to diet for the rest of their life.

Behavioural ≠ easy