I grew up in the South, a land renowned for prejudice, and attended schools which drew students for all over the city, rich and poor. I was aware than many of my friends were Jews only years later when traveled beyond my home environment and started to realize that certain names frequently indicated the person was a Jew. As I reflected on the people I knew in school I realized many were Jews and none were poor, reflecting their families livelihood. You are correct in they were a mixture of doctors, lawyers, and merchants.
The amusing thing about my memories is that I never viewed them through a prism of stereotype because I didn't know they were Jews. They were all just like the rest of us. Although in retrospect, a few did fit the stereotype, even if I didn't know it, and none of us reacted to them as such.
It's our mothers. There's a Jewish joke that goes "Every Jewish mother wants her son to grow up to be a doctor. If he's a bit stupid, a lawyer. If he's completley retarded, she'll settle for accountant."
Lots of social pressure to be a professional. That's all it is. That, and a high social value placed on academic excellence and hair-splitting. ;)