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.