Voice chat built into the game would become a defacto feature that players would be expected to use or be kicked. Of course it would also be about as useful as a chocolate teapot, so many people wouldn't use it, and the unrated nature of voice chat would be a turn off to some players, parents and younger players.

Others have mentioned the irrelevant conversion, extra music and loud argumentative chat about other topics being both annoying an distracting, along with the inevitable vocal diatribes that players will issue forth without even pausing for a moment to think better of it.

If groups want to use voice chat there are already many options for them, but because it exists outside the game it's entirely optional and opt-in only.

Last, I would never use voice chat anyway, hearing my voice speaking for my character would be immersion shattering for me. It's not so much that I role play anything when playing - I really don't, it's just that my voice would be 'wrong' and that would be a problem for me at least.