For a long time finance has been the place where the most interesting tech was going on (not to say there isn't a lot of boring bog standard stuff as well).

It's an industry where many firms have had to develop their own network transport layers, database engines, languages, etc. Where machine learning and big data have been standard practice for over a decade. Where understanding complexity and concurrency theory are important, and designing lock-free algorithms can be a daily activity. There's very few areas of core CS that aren't used in investment banks.