Perhaps bc people at Oxbridge tend to be deeply passionate about their subject, hence most branch into academia (which let's be real, isn't as high paying as banking, meaning fewer billionaires are produced) whereas people who go to LSE use their university experience as a stepping stone to get them to where they want to be (i.e. a job in finance) hence load up on internships etc. while they're there and delve into finance easily once they graduate. Like if you got into Oxbridge/LSE, you have as good a shot at getting into IB with a degree from entire university but the point here is that the statistics you see are just skewed that bc more LSE people try to get into IB whereas Oxbridge academics don't.. This is just my POV btw, I might be wrong.