Contrary to what it might look like, rowing is almost entirely dependent on your legs, your arms/back merely keep the momentum going at the back of the stroke. Also, the muscles in your back are nowhere near as strong/large as the muscles in your legs, there's nearly a magnitude of difference. Though if you added power from your arms in addition to fully utilizing your legs (it would be much harder to concentrate) then there would be a benefit. But then you run into how to engineer such a thing, especially with respect to aerodynamics.

The limit is cardiovascular, not muscular. World class rowers and cyclists have about the same power output. A rowing bike would be larger (so less aerodynamic) and difficult to engineer.