Locrian said:
As a Fermi estimate...
Your typical investment bank or large hedge fund probably has a half dozen or so astrophysics Ph.D.'s on staff and there are about 50 or so of them in the world. There are several thousand "mom and pop" small hedge funds in the world, and maybe a tenth of them have an astrophysics Ph.D.
So we are talking about on the order of several hundred positions. That doesn't sound like much, but it's a huge fraction of astrophysics theorists. One other point is that a lot of jobs in academia were created in the 1960's and 1970's are aren't open to new people, whereas pretty much all of the jobs in finance were created after 1995 or so.
The other thing to point out is that all of the academic jobs involving astrophysicists fit on one web page.
http://www.astrobetter.com/wiki/tiki-index.php?page=Rumor+Mill+Faculty-Staff
The number of jobs in finance which will consider hiring someone with astrophysics background, won't.
The fact that there are such small numbers means that it's easy to get flooded. One reason it's not a good idea to study astrophysics in order to get into finance, is that if you do that, then so will other people, and if you just increase the numbers of Ph.D. slightly, everything gets saturated.
(Also, people have this impression of banks as tiny, when most aren't. Your typical megabank will hire on the order of 100 math/science Ph.d.'s. A typical megabank hired about 150,000-200,000 employees. Citigroup hires more people than General Motors.)
One other reason that finance is attractive to astrophysics Ph.D.'s is that there is much less of a glass ceiling. In most companies the senior executive positions are off-limits to the technical people, and you end up hitting the limit when things stop being technical and start being managerial. In finance, there is much less of a barrier for Ph.D.'s to move into upper middle management where the big bucks are.