Hey, some pros also have their racquets built for them with a completely different flex than the retail version.
Good question, though: Why is everybody interested in Fed's racquet?
Well, because he's Federer, that's why. People think they will play like him if they get his racquet, because their naive. And yeah, that's also part of why they can't sell his exact frame. Even the one they sell as being his racquet is too demanding for those wannabes out there. Imagine if they sold the actual specs: those poor imitators would be dying.