I think you already know the answer well, you just want to see what people say here. 

Obesity is caused by your metabolic rate being outrun by your caloric intake.  Most of this is diet.  People consume thousands of calories a day.  Exercise can assist in weight loss, but it's usually very small.  Exercising for 30 minutes of semi high-intensity exercise burns about 300 calories or so.  In addition, exercise can often hinder weight loss because it can stimulate your appetite.  Compounding the problem, anyone on a diet will see their resting metabolic rate lower, thus making is harder to maintain a lower weight.

There have so far been many scientific studies and it's been shown that physical activity plays little or no role in weight loss.  However, it is true that combining the exercise with other aspects of your habits, like diet, can really help.  But that's mainly the diet and not the exercise.

Low fat, low card, paleo diets etc all end up with about the same results in the end.  A few of these diets performed a little better, but there was no statistical difference between them.  The main thing is total caloric intake.

Many reasons, but for the general public it's due to lack of exercise & poor diet. You'd be surprised as to how much food we consume compared to the recommended daily intake, food labels are misleading & all this 'low-carb, low-fat, low...ect.' Is bad advertisement. With a little research and a trip to the nutritionist an obese person could lose up to 4 pounds a week. In less general cases, obesity can lead to diseases & disorders which can make it harder to lose weight from that point on. Take it from a person who was previously diagnosed with Fatty Liver (which makes weight loss & activity difficult) and now is 20 pounds lighter in a little over a month. My diet change was extreme but it wasn't impossible and now I eat the recommended daily intake of carbs, fats, sugar, & protein. It's more of a lifestyle now than a diet but I'm now free of Fatty Liver & all I did was watch what I ate & did almost no exercise. Just my opinion but this is the basic gist of all success weight-loss stories

Your reasons are all valid, imo.

Here's a political one. In the 1960's the US government, via the FDA, started telling us that a high-carb diet, where we get most of our calories from grain, was necessary to have good health. There was little if any science behind this.

The energy profile of a human being is mostly fat. When you eat mostly carbs, your body craves fat, so you get hungry. Unfortunately, people often eat more carbs when they get hungry, and the cycle continues, often resulting in obesity. 

As Julissa said, the is a correct amount of carbs we should be eating. There is also a correct amount of protein, and the rest of our calories should come from fat. For most people, a healthy diet is high fat. 

Big portions of food, lots of sugary drinks and snacks between meal times.