Hearing through headphones is not comparable to hearing through an external speaker

Disagree. I think they are very comparable. Wearing headphones in a car is only slightly worse than listening to loud music in the car. Cars have gotten quieter and more isolating, more powerful, bigger, faster, and with bigger blind spots over the years. You barely have any situational awareness if you even roll up the windows, nevermind playing music in any form. The difference between a stereo and headphones is splitting hairs.