Probably because they don't care. Massively overweight people just smack of lazyness, lack of will power and no self discipline.
Sometimes. But sometimes there are other causes. Low self esteem possibly caused by abuse in the past, depression which can cause some people to over eat, a basic lack of understanding of how to cook, or what foods are nutitionally poor.
I'm never going to be slim because of my build, but I have had both high and low body fat percentages in the past. The periods where I was larger coincide with issues in my past, where I was coming to terms with stuff and suffering from depression. My self confidence, self esteem and self worth were rock bottom. The periods when I have been smaller coincide with times when I have felt positive about myself and have been happy.
I know that I am in control of what I eat, and what is good for me, but I love good food and have to work constantly to keep my weight stable. It's not laziness, will power of disipline that I lack, but the inability to keep my eye on my nutrition when there are so many other things in my life going wrong. Just like some people might take a break from running training when life gets in the way, sometimes I have to take a break from watching my diet.
What I am getting at, is that there are lots of causes of obesity and you can't lump everyone together in the same category.