While what you say is true, I would draw a different conclusion. It seems more likely that the students would prefer working in a field closer to what they studied, but the lack of positions means they had to look elsewhere. Finance knows they can find people with highly transferable skills in sci/engineering at competitive schools, so the brain drain is really the fault of the job market in the sciences and engineering.
Of course you can still look at it as finance not investing in long-term R&D such as in science and engineering directly causing this brain-exodus (as opposed to brain drain), so it's still finance's fault!
That's like a friend of mine with a math degree. He didn't want to work in finance, but despite his best efforts he could not find anyone offering him a job where he would get to use his degree. So he ended up in finance after all and gets paid lots of money and works on interesting math problems.
Of course you can still look at it as finance not investing in long-term R&D such as in science and engineering directly causing this brain-exodus (as opposed to brain drain), so it's still finance's fault!