The epidemic of people not wearing headphones has been directly caused by the lack of headphone jacks
When you speak to someone in person, you'd adjust the volume of your voice to the room and the recipient without thinking about doing it. The engineers who built the analogue phone system were aware of this effect, and made it so that you heard yourself in the handset's speaker. The engineers who designed the cell phone standards decided to ignore this so they could do more echo-cancellation.
It is not a big problem when people are speaking into a slate-shaped cell phone, but when people wear headphones that attenuates their own voice, they hear themselves less and speak extra loudly to compensate.