You still have tire, HVAC, perhaps even a little motor noise. Not to mention the ~8 kHz PWM noise for the various LEDs that dim dynamically - window switches and the various screen backlights which go in and out of phase because their dimming is not perfectly synchronized (I spent a long while with an FFT app localizing these noises). The car has fantastic NVH for a car - a subjective assessment we can agree on.
I'd want the flat response in a studio too. But in a car, I want the sound system to be purpose built for the environment, and for my taste, it isn't. I listen to music louder in a car than I would at home, and I enjoy exaggerated bass in a car to a degree I would not at home. Despite this I do think it's great that a car company had the guts to ship a stereo with such objectively flat response.
(I was one of the engineers for my college radio station back in the day. Absolutely does not make me an expert, even a little bit, but it gave me an appreciation for this kind of thing.)