Successful Producers Provide More Value To Society
Of course, we all know that engineers don't make a damn thing compared to the producers who hire them. Why? It's simple. Engineers simply don't provide as much value to a record label as producers do. The list of people willing and able to make a pro recording of a drum kit for a low wage is very long. The list of people capable of providing the listener with inspiring and commercially appealing music is extremely small. This is why successful producers are paid much, much more typically.
Most Producers Can Engineer Quite Well
In the few situations I've witnessed, the producer was more than capable of engineering just fine on his own, but simply wanted an assistant to handle a bulk of the technical work that would distract the producer from his ultimate job of creating the best music possible. So even, in the highest levels of audio engineering, you are still just an assistant. This won't bother everyone, but some of us are more ambitious than that.