Because much of that stems from how it's portrayed in media. Amputees are almost always shown in a pitiable light or with prosthetics that don't have any of the realistic limitations. Heck, in some places, artificial limbs might be considered cool. Fat kids are largely played for laughs or in other undesirable lights.

I recall a story I heard about a year ago of a grade-school girl who was missing an arm for one reason or another and had to get a prosthetic. The day she comes into class with the robo arm, suddenly she's the cool kid. The rest of her class was suddenly like "Whoa, she's got like a robot hand! That's awesome!"

Cybernetics being fashionable is practically a requirement for cyperpunk stories.