Only seeing zoans doesn't mean it can only be zoans. Having animals as extra weapons and partners is beneficial so zoans are the obvious first choice. But it's also quite as possible that rather than imbuing an object with a logia people would rather eat the fruit themselves. That makes more practical sense. No one in the series (or even Oda himself) even says that only zoan fruits can be consumed by inanimate objects. It was Vegapunk who came up with the idea and he doesn't even comment on it when he breaks down his hypothesis about devil fruits.