Why do people think that starvation diets WON'T make you skinny ?

Every time the topic of starvation diets comes up , people immediately say that they don't work . Why ? I lost 10 lbs in 5 days eating 500 calories/day at the absolute MOST , and I've been doing this for a very long time and continue to lose weight nearly as steadily . I realize that you can gain that weight back quickly , and that you reach a 'plateau' , but those are easy to get around . So why do people think they don't work ?