So there you are, trying to save the world, but no one's listening to you. Whenever someone asks you a question, one of your friends answers for you. When the game finally lets you make a choice, it's usually between a) help an old lady across the street or b) poke her in the eye and steal all her money.
And even when you pick b) out of perverse annoyance, some smartass party member will intervene, and you'll have to go through the dialogue tree again and again until you pick an option more respectful to the elderly.
So where does the silent protagonist come from? After all, the technology to give protagonists voice is still fairly young. While now it's standard for even games with very simple plots to at least include some explanatory text, lots of games have never needed to.