Indeed, SH4 was never developped as a mods-friendly game. So EVERYTHING that is modded here for SH4 had to be entirely figured out by the community: no SDK(=Software Development Kit).
Regarding your question, it's due to lack of 3d model importer (to SH4 format).
However, Skwas is working hard on it in his Silent 3ditor and I believe it's on the verge of working.
Now, i don't know for other modders but i've found a way to import 3d model into SH4 vs. some slight SH4's rendering engine capabilities lost (in fact, rendered like in SH3: light react less precisely on the model, no SH4 caustics, etc.)
If you wanna see what i mean, go check my mod "historical 4 bladed props" (see link below in my signature)
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