Why are you so picky about voices, then?
Because the voice + personality is a make or break situation for a let's play channel. In Let's plays, the game does 90% of the work. If the player(s) aren't interesting or enjoyable to listen to, then you're watching a normal person play a game, which you yourself could do. There's no reason to watch someone who speak like their mouth is full of nutella and is uncertain and unconfident about what they are going to say next when we would get exactly the same content from someone who is cool, confident, and is pleasant to listen to.
Basically, a let's play is a lot like a cover album. You don't do the bulk of the work, so if you're not nice to listen to we might as well go to the original.