I believe the primary reasons are more likely to do with distance and communication.
But there's no reason to believe that.
The idea of trying to manage a network of stores across the country when communication was by post or expensive phone calls just simply didn't make sense.
What? Why not? Cars are expensive items, phone calls are minimal by comparison. If you have an order for a car, you drop the form in the post. At least, they did back then.
In fact, the reasons are as stated. The manufacturers want to make cars at X dollars, which requires building Y cars. Right now there are cars which can't be sold piled up all over the world, for reasons like these and others (e.g. "the economy, stupid")