I wondered about this. I assumed it was because of the magic. A lot of chronic diseases were magically fixed. I don’t know that it forgives the lack of having characters with chronic problems, but it might explain it. It seems like the only people mentioned in the books with illnesses, etc. were either magically cured or beyond hope (i.e. Neville’s parents, or Lupin). And, their problems stemmed from magical. It was almost like you could “cure” any physical problem easy, but if it involved magic, things got complicated.
Just musing in type. Not a judgement either way. Although I really really wish she would have included something about this. And, there was opportunity. Even with the existing characters she had. I mean, there was a WAR. People died. People watched their loved ones die. Did NO ONE suffer from PTSD after that? There were actually TWO wars. The first time Voldemort came to power and then the Battle of Hogwarts. It would be interesting to have a backstory in HP about these problems (both chronic mental and physical health issues). I can’t believe people with physical and mental health problems don’t exist in the HP wizarding world. So, how do they handle it? I want to know! So curious!