I noticed this a few months ago, when I realized that my phone's Google navigation, all the GPS devices I'd ever used, and most of the text-to-speech I've heard in general, default to a female voice.
Why? I have no idea, but I doubt it's because we have this unconscious desire to "keep women in their place". Could it be rather that men are less likely to "argue" with a female voice than the perceived competition of a male voice? And that some women, in turn, might not like being ordered around by a man's voice while driving? Plus, as another poster mentioned, the higher register of a typical female voice may stand out better against the noise of the road, the engine, etc.
Of course, I could just be fooling myself--maybe subconsciously I just want to keep female voices in these kind of "support" positions. Who knows.