I guess it's because they try to give off a "personal" feeling, making it one of the most definitive Sci-Fi FPS games around. Not to mention, you could ask why there's no cinematics, few mirrors and such. Until long after the game's release, I never knew what he looked like. Also, in the expansions of the first game, G-Man and Freeman are always nearby but you never see their facial models.
So I guess the reason why it is because VALVe want to say to you, "YOU ARE Gordon Freeman, not Whitewalker1, etc.". Voice is just one of the few things that is hidden.