If you look at the bodies of men and women, nearly every organ has slight gender-based differences. Men usually have more muscle, bigger lungs. Women have less/no facial hair, more round faces, etc. This is normal. Evolution made us that way because in nature, males and females fulfill different roles, and we are adapted for these purposes. Why is it that people assume men and women's brains are identical for both genders when every other organ is not? Because this is a touchy topic. Because we're trying to be politically correct and not offend anyone.
If anything, the brain is one of the most crucial organ to differentiate between both genders, as it directly controls our behavior. The fact is, as StikyPad said, studies have shown that there ARE differences in the brains of men and women, which makes absolutely perfect sense. It also makes perfect sense that men and women would have different preferences for certain tasks. This doesn't make either inferior. It doesn't mean women have to be locked in the kitchen and men kept away from babies. It means maybe *on average* men prefer engineering and women prefer medicine and psychology. Note that I said *on average*. Every human being is different on more than one level.
Now, lots of people will say that if women aren't more in engineering, it's because of the mean evil misogynistic nerd men who are already there, and pushing them out. I beg to differ. Medicine used to be 100% controlled by men. It was a men's field. Women were perceived as incapable of doing it. It was a misogynistic field par excellence. Yet women MADE THEIR PLACE in medicine and now women almost dominate it! There are more female medicine students than male! Women seem to like medicine, and they proved they were capable of doing it. Soon, male doctors might well be the minority... And that's fine. Nobody complains about it.
Back to engineering. I studied computer science at McGill, in Montreal, a very liberal Canadian city. This university prides itself on acceptance. Women were the minority, as they had always been in the field, but the ones who were there seemed generally happy. The guys would have liked nothing more than additional female students. They just weren't joining the program. Furthermore, the women who were in CS, at least all the ones I met, were always more on the mathematical side of things. They all seemed to generally dislike coding and avoid it as much as possible. Could it be that there's something engineering-y about coding that women usually tend to like less? Could it be that the mathematical side of computer science tends to appeal to women more?
I think it's a shame we can't openly talk about these possibilities without being labelled sexist. Shoving our heads in the sand and pretending women and men are exactly the same on every level isn't scientific. It doesn't match everyone's daily life experience... And it isn't what feminism should be about either.