This is because people's perception of overweight is wrong and distorted by the increase in obesity. The classification made by BMI wasn't invented by a shitlord doctor to make people feel bad and promote the diet industry; it is based on the researched levels over which someone's excess fat is...
It's around 35% of school age kids that are classified as overweight or obese as far as BMI is concerned. And BMI is a good indication of the point at which weight is likely to cause health issues.
Edit: Indurains Lungs beat me to it. However another point to add is that people often don't...
We should also ban use of the word big when describing someone who is fat. And stop giving out blue badges for fatties as well; they last thing they need is to be able to park right next to the supermarket.