Because if you put "Owned by an 18yr old teenage bloke who has all the Fast and Furious DVD special edition DVDs and likes listening to loud techno music", would you buy it?
I agree with the general thrust of what you're trying to get at here, in our progressive, modern, non-sexist society it shouldn't make a difference. But the reality is that by listing "Lady Driver" it invokes an image in the potential customer's mind of a vehicle that's been taken care of, serviced regularly and not driven to within an inch of it's life.
NB - That image may not match up to reality - most of our preconceived assumptions don't, and I've no problems admitting that I've been spectacularly out-driven (and out-ridden) by a number of girls in the past. When I first started at my current job years ago I became good friends with one particular girl who's daily-driver was an older Celica with an engine swap and a full roll-cage and harnesses. It wasn't a good car by any stretch of the imagination, but man was it quick.
Thanks!
Matto
(Who's previous list of vehicles includes, among others, a '77 Charger, an '84 beige Sigma that could push just over 130db, a 1600 VW beetle and a Nissan Silvia. Something for everyone in there. None were sold as "Lady Owner", btw.)