You can not compare a road vehicle gearbox to a CSU on an aircraft. Part of what you say is correct regarding optimal RPM of the engine, but you also need to achieve the optimal aerodynamic benefit from the propeller. This is achieved from the best prop speed at the most efficient angle of attack. Too much angle (fixed pitch prop at low forward speed) and the prop effectively has stalled sections and increased drag. Too little angle (prop at high forward speed) and the prop can not generate any more useful forward thrust without increase rpm, you then get into further issues of tip speeds etc...
The only way you could replicate this on a road vehicle is to have variable tyre size so that the optimal wheel speed is achieved at the same time, ie as speed increases the tyre grows and wheel rpm remains the same. This would be the equivalent of an aircraft CSU. However this is just not required for a road vehicle as the tyres road speed does not impact greatly on the vehicle, where as propeller speed and angle is essential for efficient aircraft operation.
Even with an engine that has constant torque at all RPM a fixed pitch prop will still only have one rpm/forward speed combination that will provide maximum acceleration. This means you can set it to perform well on take-off and climb or achieve high speed flight, or, a compromise somewhere in the middle that doesn't really do either well.