If they want fat people to be represented in comics (god knows why), why don't they ever just give one of the existing heroes an eating disorder brought on by years of PTSD from fighting villains and seeing civilians die that causes them to overeat and gradually become overweight to obese?

Absolutely nobody likes characters like "Faith" which tries to spin obesity as something good. It should be shown as what it is, a problem, and some random nobody that you've never heard of doesn't work because you have no stake or interest in seeing the character get healthier.

It was kind of featured in Spiderverse but only to be made fun of and played for laughs.
Imagine Superman having to sit down with Aquaman and have a talk about his weight.