[2dollars27cents]

There are a few reasons I could think of:

1 - Most industrial DMLS/EBM systems have a build envelope far too small for this. A system like the Norsk Merke IV might have the required size, but parts built that that process still require post-machining. 2 - Tolerances. Casting -> Machining still offers greater control over the final geometry than additive processes, especially at that scale. 3 - Casting is a bit more design-agnostic than additive processes. 4 - QC processes for casting + machining are far more defined than for additive processes.

None of these are insurmountable challenges, so I'm sure in the future a laser-sintered/EBM/plasma-deposition process will be used or at least heavily-considered.

[JshWright]

What would be the advantage? "3D printing" is great for geometries and structures that don't lend themselves well to traditional CNC milling, but a grid fin is a really straightforward part.