> If such a drug exists, I want to know why the body hasn't evolved either to produce it by itself or to obtain the same effect in another way

You're assuming evolution is perfect and that we have reached perfection already

Your question is also assuming that each drug impacts everyone the same way

Some people who take a certain psych med won't experience side effects, while others may experience severe side effects

E.g. most people who take Straterra experience benefits but some outliers may become extremely suicidal

I would say overall human science on drugs is primitive. I think to achieve what you are really thinking about will need a different overall approach to psych med development

I wouldn't overlook the potential for naturally found compounds either. Many indigenous peoples have or had vast knowledge on their pharmaceutical effects but often these were destroyed in genocides