I can think of several reasons you don't see overweight people at the gym.
1. A lot of people who are overweight are eating cheap carbohydrates because that's all they can afford. Expensive meats and fresh fruit and vegetables cost a lot more than potato chips and hamburgers. They gain weight because of the carbs, and can't afford to go to the gym to get the weight off. Sometimes they can't get a job because employers don't want to hire overweight people. It's a vicious circle.
2. A lot of the equipment isn't designed for big people. It's uncomfortable. Look at the woman in the photo above. That seat isn't designed for someone her size.
3. Most exercise outfits are also not designed for bigger people. It's embarrassing to go to the gym in non-gym clothes. It's also not effective to try to work out in blue jeans. You need clothes that can stretch.
4. I think the biggest thing is walking through the door. Fear of failure is very, very real and not something to take lightly. I wish people were kinder to each other.