Because food, specifically sugar, is addictive. It's not an 'excuse' but it's perfectly understandable when you see people who don't lose weight, because fast food companies and unhealthy snack brands shove advertising down everyone's throats, when P.E. is one of the most useless time wasting lessons in school, when the government won't get a grip and just force companies to make their products healthier and
most importantly, when people who are so dim-minded like you, think that the solution is to tell them to 'just lose weight' and thinking shaming and mocking someone will help them do this.
Why people think shaming/mocking someone, who uses food to comfort themselves, will help is beyond me.
Society did not end up with all these overweight people overnight and they will not simply go away overnight. Companies such as McDonalds, KFC, Burger King, Nestle, etc. are leeching off of these people and are only but encouraging the current obesity epidemic. As long as they have such a large amount of power and control in this country, society collectively will not get thinner.