It goes right along with all the other expectations women have of men in a relationship. Either you let him be the man and treat you like a woman (take the risk to approach you, ask you out, pay for your dinner and movie, etc.) or you need to throw ALL of that out. You cannot have it both ways.
Because we look good with one hand on the wheel :P It's a confidence boost to drive ya lady :D
Seriously, it's either because the man should "take care" of her, thus doing this task for her, or cause society still has the impression that men drive better than women. (But then men act wreckless and confident and cause all the accidents... but that's none of my business.)