I don't know if earphones were invented for the benefit of other people, but to hear the sound better. I'm getting a picture of war-time radio operators all in the same room with headsets on. They really needed to hear their radio only...
I find that the traditional earphones, the solid ones that sit in your pinna, leak way too much sound. Inner-ear earphones can be much closer to your eardrum and prevent back leakage. Sound doubles in intensity if you halve the distance to the source, so inner-ear earphones may not need to be as loud, because they put the source marginally closer to the eardrum. The rubber also directs sound directly to the eardrum too, meaning less lost sound.
Remember Apple talking about how the revolutionised the earphone? Traditional earphones do not sit well in the ear, pointing a lot of the sound in the wrong direction, so they moved the speaker.
So, reason one: earphones are poorly designed.
Reason two probably is that they just don't care, not that they don't know. I'm obsessive about the external volume of my earphones, but will crank it up to full volume if the ambient sound masks the leakage.