Because dealers and manufacturers need to transport cars in bulk, at regular intervals, all along the same route to the same destination. Using a freighter only makes sense if a certain number of ships all need to be transported along the same route in the same timeframe, which doesn't seem likely in most situations, considering how low-traffic most of the inhabited galaxy is. I'm okay with a delay, I just don't think it should take longer than it would have taken to just fly the route myself, 1st because it's wildly inconvenient, and 2nd because it's not realistic. It's like the worst of both worlds.
Fair enough, I was actually pointing out that it's not unusual to load one vehicle into another vehicle despite the first vehicle being able to do the same as the second vehicle but faster.
An example would be .. why do some people load their car onto a train when they are moving between cities.. they can take the car, it'll likely be faster and cheaper .. so why do it?