Complexity Over Quantity.

Legion chose a world beast for several reasons. One of the reasons is for the inner world. It has granted them a very powerful Origin domain and it has served as a domain for Legion-9. The most important reason is still the fact that the core can be assimilated into his existence without issue. When he possessed the core, it created a new body for him immediately. 

His situation is very unique. He is not only different from other Origin gods but other Origin god-level world beasts aren't like him either. They don't have a separate origin soul in their inner world like him. Their soul within their inner world, their body, and Origin have become one. He on the other hand has a separate Origin soul and an Origin body.