On paper you are right. You can file an NDA. But in reality you can't prove that the product on the market (the drug you are adding an NDA to) is sufficiently under your control to enable you to claim that it is safe and effective. Only a manufacturer can control the product. If you want to piggyback on top of an existing product in the market, you have to become a manufacturer or sign a contract with an existing one.

You are absolutely right about the incentives. There are absolutely none except to help humanity which is why only hospitals and universities are doing these off label studies.