The rotation of the engine argument just doesn't make sense.
As Craig said, most of the boats use standard rotation engines which would do the opposite if there was only a driver. A few boats, such as Correct Craft (there used to be others) used reverse rotation engines.
Any competition ski boat would be designed for a driver and one observer. For the situation that really matters the passenger weight is balanced (unless you want to make assumptions on the relative weight of the driver verses observer.)
Competition boats all follow the same layout because it is pretty much dictated by the conventions of competition slalom skiing. What side the driver was on might have been a coin toss back when the tradition started, but once it was established it wasn't going to change. A serious slalom skier is going to buy a boat that is approved for competition. A casual slalom skier will buy a boat that at least has the pretense of being a competition cabable boat.
Many years later wakeboarding enters the scene and they start making "cross over" boats. Intended for both slalom and wakeboarding they followed the slalom boat design. When they started making wakeboard specific boats, why change now?
Get away from water skiing/wakeboarding and you will find a lot of boats that are configured with the steering on the left or even center consols.