Aniplex doesn't "own" anime, the first owner of anime are the creators and authors. Who very often are the writers of light novels, novels, manga and so on, simply because they are the ones who create the original thing. When the anime itself has not "source material", then the direct creators pass to be the ones involved in the anime studio, while ownership is more complicated to determinate. While in many cases Aniplex has the distribution of some anime, they're not the only one producing it (let alone making it directly, which is on the anime studio).
Good for us, Japan doesn't replicate the way of doing things of the West. In which you will probably be in the right and one company will own almost everything, you like an author will need to comply with whatever crap the company in question comes up, or be shunned.
The way in which Japan does things is not the optimal of the ways, I think it should be more descentralized and more "indie". But it still far away from the nightmare that is the West right now. And that's one of the reason why they have anime in the first place.