>>55429512It's because it wasn't economical.
You battle your rivals like 5 times each so making a more detailed model for them is fine.
You battle each gym leader and E4 once and none of them appear in any major cutscenes, so giving them a more detailed model that's used in battle intros would be "wasteful"
If anything they'd probably consider giving generic trainers battle models before the one-off boss NPCs.