Because MGM+ is a seperate business that they bought and they are leaving it as is. Just because they own both doens't mean they have to merge both. In the same way if I owned a gym and a yoga studio I'm not going to force everyone to only use gym membership to access both when not everyone wants the gym membership. And if it doesn't help the gym business enough, I'm not going to make the costs of my gym business even higher by adding additional costs to it.

They aren't taking advantge of lack of knowledge because them owning both means nothing. It's a business decision if they want to merge products.