You can set up the speed of the auto function, so I don´t how that has anything to do with voice over.
Alright, since you need it spelled out I guess I'll explained it in as simple terms as possible...
Auto mode is pretty much everytime cattered to voice-over. When the audio finishes, it goes to the next line, allowing the player to follow easily. With no voice-over, a time has to be specifically specified
for each line. Because giving the player two seconds to read 5 lines worth is going to be too fast, but good enough if there's online 5 words on screen. On the opposite, having 40 seconds for the first one's fine, but will leave the player waiting impatiently for the second. So it can't be programmed as an overall, general setting.
Then comes people's individual reading speed. Some people read faster than others, some people read slower than others. Some people's native language isn't english and sometimes they need a little bit more time to understand a specific sentence. Auto-mode can't account for all of those without voice-over to carry a specific rythm.
So that's why most devs don't include an auto-mode if their game doesn't have voice-over.
Not even counting the fact that a simple click or button push isn't going to drain your energy.
