Is obesity really genetic?

Why do fat people keep on suggesting that obesity is genetics? Sure, some people might have a slower metabolism them others, but this doesn't mean that you can't break it even by excercising. Everytime I ask an obese person if they've tried to diet and excercise, they tell me they did, but gave up because it didnt work...DUH!!! ofcourse its not gonna work if you give up. JEEZ, you need to keep on striving at it, it takes months if not years to lose all that blubbering fat, so why do fat people insist that it's genetics? Why can't they give up their laziness and stop giving up and actually work out and diet until they meet their goal, and stop ordering pizza everyweek and eating out so much???