In most cases, voice acting doesn't really seem necessary in a game, and can be distracting.
Cases I've seen it done well are mostly commercial releases.
I feel like it's main force is to round out animation.
Games where a certain level of character animation is implemented, especially in the cases of fine details like talking portraits and cutscenes, often are best done with voice work. It's mostly a result of high production values in games. If a characters mouth actually moves, it only makes sense that some sound should come out of it.
But in 99% of RPG Maker or other low production value games, it largely seems unnecessary, if not distracting. If you're not gonna animate the characters mouth when they talk, why bother making a sound come out of it. If anything, it's weird and breaks immersion.
There are notable examples where voice acting plays a part in the sound design, but that kind of sound design probably isn't what you think of when you say voice acting.
So take it as you will. This is simply my analysis, and up until now I haven't given much thought as to why games should or shouldn't have voice acting.