It's true, women do get married young and end up leaving the medical profession, but really in Pakistan, medicine is really the only noble profession that women can go into. What else can they really do? They pick medicine for higher education much for the same reasons that men do. Sure there are doctors out there that really want to help people, but I wouldn't say I'm one of them. I'm down for helping people, but my main motive to pick medicine wasn't the philanthropy. If females don't end up practicing it's often not their fault, since society really demands that brides be between a very strict age limit. It's obvious that women have different roles in Islam, and childbearing is one of them, so naturally for all aims at a healthy family, people do want to marry younger women.
I also agree that those who really do want to practice will find a way to get married young, finish their degree, and take a few years off while they have kids and such, and then get back into the work force. If this wasn't true, there wouldn't be any female doctors at all. Society however has made it very easily acceptable for women to simply drop out or finish their degree but never practice, partially because of the male dominance but also equally in part to the laziness of women who feel they've been out of the rigorous medical profession too long and that it would simply be too hard for them to take it up again.
The reason that men say they want to marry doctors is because they know well enough that their wife (or wives!

) will never work, but they want to assure themselves, as well as anyone else who asks, that their wife isn't a dumb***. Being able to claim that she's a doctor quickly shatters that view.