And building a real show car and a model car are also two completely different things.
It's very rare that one single person has all the skills needed to build a car. He/she would have to be an engineer, a welder, an upholsterer, a painter, and on and on. Obviously there are very few, if any, real show cars competing for awards that are completely built from top to bottom by one person. The assumption is that the car is the result of a team effort. The Ridler award and similar types of things are a judgement of the result of the collaboration of everyone who was involved in building the car... not so much the talent or skill of the individuals involved, but the finished product itself.
A model car contest is a completely different thing. The builder's skill is being judged, at least as much as the finished model. It's assumed that one person with reasonable skills can build a model car, and that idea–one model, one builder–is what every model car contest is based on. Submitting a model that you had help on is cheating, plain and simple.