“PHds in math don’t usually work on wall street.” ?
Even 30 years ago, there were lots Ph Ds in Math (or STEM) in Wall street firms, leading transactions like derivatives, CDO, MA etc. Now you are able to find Ph D holders in their back and middle offices. The government requires Wall street firms have one of these guys in their board members.
“Financial Engineer” is the one of specialized area, usually under Math, Statistics, OR, or Industrial Engineering departments.
The career path for newer generation coming in to Wall Street without this background would be very limited.