Have a sore throat? You could probably play that game in text option.
I'm sure written dialogues aren't hard to programme. And the whole point behind voice recognition technology is that it can recognize your voice and tell you apart from your annoying girlfriend
I don't think realism is the aim of every game developer, nor should it be. Some games make a concentrated effort to carry realistic identifiable traits of our every day lives and implement it in a completely alien world just so we can believe in that world more.
With your argument, game series like Metal Gear Solid or Final Fantasy wouldn't have to bother themselves with something like the development of character facial expression just because it's being done for the sake of being realistic. Yet, I'd wager most people like it because it makes the story all the more enjoyable, the world more lively and the characters more interesting. You can be pulled out of a sci fi movie because of shit actors. Likewise, people care more for a story if the game is more immersive.
Not every game wants to be realistic. Some games just care about gameplay while others try to create a cinematic experience. This technology could really help the latter.