The banking sector hires top engineers because of there unique problem-solving and analytical skills which are otherwise missing in business undergrads. Engineers prefer banking jobs because they often pay more.
I see a win-win situation here. Not sure what you really want to know?
If your question is why don't engineers go for a job that is more suited to their degree, the same question could be reversed and asked as to why do banking companies hire engineers when they can hire business undergrads. Life is not a movie where everyone follows their passion. Money is a big pull for a lot of engineers and that is why they choose banking.