When you use your car’s built-in speakerphone feature, the speakers in the car door reverberate, sending the sounds outside of the car. Maybe don’t do this while your car is parked. Everyone in the neighborhood or store parking lot can hear your very intimate or business-related conversation.
My neighbours have long conversations in their cars. I can hear every word the distant party is saying.
I think the audio levels get set higher in cars to combat road noise and the missing sound spectrum of phone calls, because it sure is loud.