The car radio has less focus on fidelity, therefore through a variety of measures including narrow selectivity, high sensitivity and making the signal effectively mono at HF it gives cleaner sound in areas of poor reception. Most hi-fi tuners employ wider selectivity, lower sensitivity and wide channel separation and this requires a better and stronger signal.
Wide stereo separation, low noise floor and very low distortion are far less important in the noisy car environment and so the technical compromise can be usefully shifted to making the tuner better with poor signals.
You can replicate this effect on a home hi-fi tuner by switching to mono on a weak broadcast - everything gets cleaner and noise drops.
If you want the best of both worlds, seek out a good top end Japanese tuner from 1975-1985. Pioneer, Trio, Hitachi, Technics and, the best IMO, Accuphase all made stunning tuners able to make the most of any conditions.