You don't have to be a shrink to know about sexual
perversions.
Just like you don't have to be a brewer to know about beers.
Neither do you have to be a mechanic to know about cars.
If we apply your trend of thought, then Nigella Lawson shouldn't
know anything about food cause she wasn't trained in Le Cordon
Bleu, which, as we know, is not the case. (I don't think she
attended any proper cooking school, just like lots of other famous
chefs.)
And no, I wish that all my patients were sedated and quiet but
alas, that's not always the case and in my current job, it's never
the case. 
