When you see behind-the-scenes shots from ADR/dubbing stages, they're almost always pitch black, except one or two sharp lights.
Why is this? You'd think it would be more comfortable to work in a properly lit room?
Both as a necessity when using projectors, and as to not risk distracting the actor. Mind you, voice-acting is MUCH harder for actors then regular physically based acting as all there is is a screen and a mic and nothing else. Dimming the room helps them, in my opinion (I too do a lot of voice-acting as well), to forget where they are to some degree and imagine doing what they do on screen.
Personally, though I have no problem acting in bright daylight (other than that I have to use shades due to an hypersensitivity), I hate voice-acting in bright light. It annoys me, and that shows in my result. The same applies to many of my costumers, even many directors. No doubt, cinemas would still be dark if they changed to humongous OLED-screens for some reason. Though sound is the way to give a movie life and credibility, undisturbed vision that makes sense is the way to keep the audience focused. Absence of sensory stimuli is never a problem as long as there is something that we can focus on, in this case the movie, but unwanted impressions can be very very distracting. Ergo, it's best to keep EVERYTHING not needed for the scene as far away as possible. For people suffering from different dysfunctions in attention it's even more important, and many of us in this industry do.