Currency isn't just money. It also represents the manpower and essential ship components ready for use. A ship can't be expected to be fully constructed in a vaccuum. Parts may be constructed on world, shipped to space, then assembled at the shipyard. In this way the planet bound factories get used to the fullest, and the shipyard does everything that can't be done on the ground. In addition, a space worthy crew needs to be trained to properly use a vessel. Currency also represents this essential resource. These kinds of materials have value to everyone else that also wants to build a space worthy fleet, hence it is a stable form of currency.

So pretty much, currency is abstracted out to be everything that isn't essential elements or high value materials.