Why are Jews so heavily over-represented in intellectually-demanding occupations?

The reason is rooted in history. Because European Jews have always been a minority in whatever country they called home (until the founding of the modern state of Israel), they went through centuries of oppression. Laws were ped limiting what Jews could do and not do. Some were literally forced into being jewelers and brokers of precious metals and gems (the origin of modern banking) because church law forbade Christians from engaging in such. Of course the tradition of scholasticism had an ancient origin in the study of the Torah, but also recall that many Jewish families had experience in moving from one country to another seeking better and more tolerant living conditions. This made for a people more sophisticated than those in authority over them. Many doctors during the Middle Ages emerged from the ranks of well-traveled and educated Jewish families. By the 21st century, the tradition of scholastic achievement is very thoroughly entrenched within Judaism.