To be fair, I also agree stigma is probably the biggest problem which keeps incidence figures low because people are ashamed to seek help.

That said, I am not so sure about all cultures being equal, in that respect.
The smaller the number of people getting help, the bigger the stigma. Look for example at unipolar depression.

Much, but certainly not all, of the relative acceptance of episodic unipolar depression is because there are more people afflicted, compared to us (according to some figures maybe as much as 10 to 1 or more).
There are people who want BP-III for that very reason (which, personally, I think is clinically useless).

Childhood trauma doesn't have to be PTSD-like severe. It can be very mild but still make a big impression on a young mind. Of course, there is no reason to think it has to be there. Psychoanalysis luckily doesn't take centre stage anymore when it comes to psychotic disorders (or anything really). To me it seems if there were to be one person who was chronically delusional, it would likely have been Freud.