I think, as the replies have proven so far, that there's no single reason why obesity is such an issue. I think it's a combination of everything that people have said here, and for each of us, one particular factor may be more prevalent than it is for someone else.
Emotional issues. Comfort eating. Boredom. The sense of "treating yourself".
Price. I can buy a microwave ready meal at a fraction of the cost it would take to buy the individual ingredients. Yes, some healthy food is cheaper but not always.
Time. Modern lives are busy - we grab processed snacks such as chocolate bars and crisps to keep us going. Little time to prep a meal on some days means if you haven't planned in advance, you're probably going to eat rubbish or get a takeaway.
Lack of exercise and movement. Everyday lives in general are far more sedentary. I'm scared of lifts and there's virtually no-one who takes the stairs with me.
Unhealthy food tastes nice. Sometimes it's no more complicated than this.
Alcohol. This piles on the pounds even if your diet is relatively healthy.
Poor health. Lack of available healthcare/long waiting lists often leaves people in chronic pain and without the capacity to manage exercise. This includes lack of access to mental health services.
Cost/shame. Gyms are expensive and committing to a contract may be daunting. If you're especially large you may feel embarrassed to go to a gym.
Safety. If gyms are a no-go, why not walk/jog? Again, potentially shame - but also safety. If you work then before/after work may be the only options when it's dark. As we know, safety is a very real issue for many women.
I could go on. Not all of the above will be true for everyone that's overweight. Some will apply more than others. But in varying degrees, those are just some of the reasons. In my experience, it's rarely down to poor knowledge - most people know the basics of what is healthy. The government seem to be taking the approach of just making surgery/unhealthy stuff more expensive - and that's never going to be a solution as cost is only one small element of it. A heavy-handed approach is just going to get people to rebel and make them defensive. A carrot rather than a stick is needed to help people make healthier choices.