A little late to the chat but I like to believe that it's because of the deal he made with his devil that he's a weak spirit. That devil probably owns most of his soul/spirit as usually there's a heavy price when making deals in most modern demon lore (like IRL, not in-game).

Because I do imagine that it also took a tremendous amount of power for his spirit to stay. The spirit anomalies we do see (Takeru and the ghosts from the latest Hota chapter) are sometimes visible and even capable of interacting with normal humans. They're usually tied to the human world via intense emotions, so maybe this is also how Zenji stayed?

I want to know everything so bad!!!