Not difficult, but can you close an investigation on that note without going over other possibilities?
What if there's another safety lesson to be learnt here?
The investigation isn’t closed. A preliminary report is usually required to be released within a few weeks of an incident.
To answer your question: because it is a very rare occurence.
It's not that rare, and there are institutional factors (such as seeking treatment for psychosis being career-ending for a pilot) that incentivize serious pilot mental health crises being untreated.
All commercial plane crashes are very rare occurrences.