Because the results are contrary to the expected outcome. It either means they are eating too much of a healthy food and the calories they are consuming is equal to, or greater than, the amount they are burning or they are simply not exercising very hard.
Believe it or not, there's a "right" way to lose weight. Taking a walk or a light jog and eating "healthy" food isn't all there is to it. You can eat healthy, sure, but if you're eating peanut butter with your celery sticks, two scoops of peanut butter adds 200 calories. Eat 10 celery sticks, assuming each one as accompanied by a single scoop of peanut butter, you just had a 1000 calorie snack. I know this because my little brother is one of those people: Once we got him on a meal plan and exercise routine, he's lost 50 pounds.
The skinny girl who weighs 100 pounds and eats 4,000 calories of junk is one hell of a far-fetched anecdote, as well... Nobody eats 4,000 calories of junk without gaining weight. These type of people typically gorge themselves one day, and eat very lightly the next day or two. It's just something they've done all their life, so it's easy to say they're always eating when they really aren't, or are eating very little. I know: I'm one of those people. I easily gained 40 pounds since I started a meal plan and started working out. I've been off my meal plan for a few months and have been bouncing back and forth between 157-162.