Because, first, not everyone has a modern computer, and second, every little bit of performance helps. FSOpen is not a wildly efficient engine, just because of its age.
Sure, the little bit of performance you save by LODing your model might not mean much on its own. But if that model is placed in a really big mission, at a large distance from the player, along with lots of other ships, then it's really helpful if all the ships in the missions have LODs.
Steve-O's Hyperion (the Karuna from BP) doesn't have any LODs, and it makes mission design with it a bit difficult, because a Hyperion really far away is just as taxing as one right next to you.