It's a communication problem. If an American sees "paella" on a menu in Spain, they're not going to recognize what's on the plate. It's not that Americans did or didn't invent it, it's just not what they thought they were asking for.
An American would absolutely recognize it for what it is, it might just be a bit different from what they where assuming.
That being said most Spaniards happily chuck whatever they happen to have laying round into their paella when cooking at home.
Interested in how you came to that conclusion. All of the restaurants near me that serve paella offer a wide variety of protein options.