In addition to what's been mentioned, printing has much tighter tolerance requirements, e.g., to distinguish between "h" and "b" at a small font size. It actually has to be a very good spatial representation of the design. And how cheap printers actually accomplish this with crude bits of plastic and stamped metal is pretty amazing.
On the other hand, 3d printed objects may only need to satisfy overall mechanical needs, or be suggestive of the shape that they model, to serve a purpose. Most 3d printing that I've seen needs a bit of hand tooling at the end, to really be useful.