I have read through all the answers but haven't found the answer. BkCreative you have asked very interesting question and one of true answers I could provide could be because people are willing to pay for the brand. For example, if Hyundai manufacturer would put BMW symbols on the car would people buy knowing the fact that it is not original manufacturer made?
It is also up to us whether we want to keep the symbols of the car. There is no law saying that owner of vehicle should keep the original signs on the vehicle. This way we don't have to advertise.
Also people keep the symbols to show their class. The more expensive vehicle is the high the class the person is at.
Is this the answer you're looking for or you want to hear something else?
People don't care... You have thought about this and it made you think. But what about others? Do they think about such things? People don't care and manufacturers do, that's why people pay and manufacturers get the money and free advertising.