Because you can’t hear yourself through the earpiece
On a regular land line, your voice comes through on the earpiece. If you are talking too loud, then you know because you are hearing yourself though it. Kind of like the way you know you are yelling too loud when talking.
On a cell phone, there is no return of your voice on the earpiece. Not only can a person not tell how loud they are talking but their voice is being muffled by the earpiece on one side so they speak louder to sound like they normally do when speaking in person.
Thats my theory anyway.