What you have is a game running on an engine that was made before half of this forum was even born. You have the names, and the models, and even some decent physics but this pack is not representative of something you "Already have". Because these things will be running on a much more modern engine, with much more modern features. Like as mentioned VR. Like as mentioned weather. New tire models. Different physical simulation. It's not the same thing.
Nobody is forcing you to buy the Reiza DLC. But it's a strong business move for them. They are branching out in a niche market in order to build bonds with competitors and share in the stakes of those bonds. They're offering updated content to a new system. You can view this as content you already own, not buy the DLC, and live a happy blissful life if you want. Preferably without whinging like a spoiled child on the internet about how your daddy company is having an affair with somebody you don't like (while you're in the house of that somebody. That's going to get you REAL far.).
I for one, am very excited to see what results this collaboration produces.