The issue is coming first from the lack of assets. Voice samples that you can use for speech synthesis are not available easily, apart from your voice and a few friends' maybe. Then there's always been the issue that speech synthesis was not very good (we had working speech synthesis back in 1985 on the Amiga Workbench, but it was pretty much what you'd expect in terms of quality), and "not good enough" can break the experience.
On top of that, speech synthesis is not an easy problem, it's definitely in the realm of machine learning, and video games development does not touch so much on machine learning so far. I think there's a ton of applications for machine learning in games (such as making less stupid AI, or more human-like AI), and speech synthesis is definitely one of them.