Frankly, I truly find that the majority of people don't know or care about sound quality period let alone in a rental car.
For instance, I ran an informal survey among about a dozen family members on their listening habits, car and/or home. Frfr, only two had any preferences. I am one of two that actually has a kit, out of six siblings. My brother has a decent HT he uses for music as well. Nowhere near my modest one. He definitely has a bit more bass and treble than I would but...
All that to say that its not just rental cars though you are spot one about them. And what some other posters said.
Ever get in the car with someone and the music sound quality makes you cringe but they have it turned up loud because they think its sounds good(and it probably does for them)?