I remember having the same question and also being disappointed you couldn't easily transport them, so I ended up using SM mode to simulate transporting them.
However, the RP reason I read and always appreciated is thinking Financial Centers more along the lines of Financial organizations and infrastructure developed on your planet. Distributed trading floors, companies, offices, credit tracking, internal trade arrangements, etc. Things you can spend government funding to specifically develop, but not something easily boxed up and put in a cargo container.