A couple of theories from me here, no real evidence to back any of it up:
Lots of cars have some form of permanent carb-heat. Many cars have the air intake placed just behind the radiator, for example. Some cars, such as mine, route some of the coolant along the inlet manifold (which helps vapourisation as well as removing ice that forms in the manifold itself). In many cases, the air intake position can be adjusted by the driver, the idea being to position it behind the radiator in winter, but somewhere cooler in summer, although I don't know any drivers who do this.
This would not be acceptable in an aircraft though. Pilots don't like being robbed of power, especially on take-off. Then there are certain conditions where it's far too cold for carb ice to form, but where the application of carb heat for any length of time may warm the carbs to carb-ice temperatures. And there's also the fact that pilots like to know what's going on - if an engine is subjected to carb ice conditions, a driver will continue to drive on obliviously while the warm air coming off the radiator keeps the carb throats clear. A pilot, on the other hand, would like to know that carb ice is beginning to form, so that he can climb to cooler air, descend to warmer air, or possibly land.
Interesting thread!
FFF
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