I guess it's because it's statistically more likely that someone has high cholesterol levels when he is obese or that someone has anemia when he is thin.
Sadly because it's comfortable and easy many people like to oversimplify the world and just assume that characteristics that occur more often within a certain group must be true for all members of that group.
It is simple thinking. People have the social habit of blaming other people as soon as they regard them to be outsiders, especially off norm when their appearance isn't mainstream. Human societies push non mainstream individuals to the fringe of society and blame those for being on the fringe and blame it on self inflicted reasons. The pattern is like this: You look non standard. This is a symptom. We don't want you and your symptom. Therefore we push you outside. As someone on the fringe you are an outsider, but not we who pushed you there are to blame, it is your own fault and it is not responsible of you not to adapt. You look like a skeleton? This looks as if you are sick. Or it looks like you do not get enough food. This feels like accusing us. But it is your lack of health which you didn't care for. So die or get out of sight, but do it silently and invisibly somewhere but not here.
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