I think designing a system like that that gave the control necessary and worked predictably would be difficult. I'm not a navy person (the closest experience I have is driving fishing boats on a lake), but I do know that steering a big boat is complicated.
Steering via the rudder, via controlling the different throttles, or some combination has different characteristics. And which one is best would depend on the ocean and weather conditions, overall speed of the ship, as well as many other factors I don't know about.
A computer is unlikely to be able to properly account for all of this. And if it did, it would affect how the ship controls, something that you really don't want to do on a large ship.
Combining multiple controls into one computer managed control can be a good design. But I think it's the wrong design for large ships.