I just don't get it - if it's so hard to eat 1200 cals, then why the weight problem?

Ok, I'm not talking about posts from ED recoverers or sufferers - for them I understand that 1200 feels like a LOT of food. But for the people posting that are overweight and struggle and have to STUFF their faces to get 1200 calories - why are they overweight to begin with? I would think that the majority of people gained weight from overeating and that yes - undereating can certainly hinder weight loss progress, I wouldn't think of that as the root of the problem. Yet it is a recurring problem among people on the forums! They just cannot reach 1200 calories! I wish I had the problem of having eat more calories in a day :) I struggle to cut to 1500 (which is the minimum for me as I'm a teen girl. And yes it's enough, I'm 4'11" and petite) and sometimes 1800 on work out days. It's a CUT. 

Could someone explain it to me please?

I have a question... it's out of actually wanting to know not an eating disorder!

Why do we actually need a full 1200 a day if we are taking in enough nutrients and supplementing with vitamin pills if we feel absolutely fine and are functioning properly on less?