Because one starts with nothing, 3D printing is a very different process. With traditional methods there is normally an expensive tooling step e.g. a mould, patterns, jigs, CNC programming. 3D printing has no investment cost.
These differences produce a new financial model. With traditional methods one starts with high unit cost but once high volume/mass production starts, unit cost plummets.
In relative terms 3D printing starts with very low unit costs but as production starts unit costs essentially remain the same, failing to provide the advantages of mass production.
3D printing can therefore be hugely beneficial for prototyping both from a unit cost perspective and in terms of turnaround speed.
3D printing can offer advantages for low volume production – refer to the "Case Study" for an example. For a part, say the size of a USB stick, we would expect to produce several hundred and provide a cost advantage over tooling etc. An automotive 12v battery casing would probably be in the tens not hundreds. It is fair to say, given our experience, that once one reaches a thousand units then 3D printing is unlikely to be competitive. Ultimately of course it depends on your investment cost to get to production.