Self-employed craftsmen are entrepreneurs with corresponding risks and earning opportunities. If they don't make their 5-7k net with a well-running construction business, then I wouldn't even consider building with them at all. Then they really wouldn't be particularly talented. And they can register the car under the company, so pay gross and also deduct the costs for tax purposes.
And let's just assume, as is often the case, that the company owner is "simply" an heir/successor in the company. All initial investments/risks are then eliminated, and they practically earn their entrepreneur salary at 25/30. They build their own house cheaply with their contacts from the construction industry, and why should they spend much otherwise? Well, a car for example, cheaply registered to the company and apparently it still makes quite an impression.