It's common for men to be lambasted for our percieved shallowness. But I was thinking about this last night and if I'm being brutally honest I have to admit the conclusion I arrived at after much objective thought was that women care more about looks for two main reasons.
1) Men have a wider range of what they consider to be attractive in a woman. Even frumpy types in long skirts (the real librarian type, not the hipster type) are attractive to some men. There seems to be a fetishist for every imperfection too, barring say... physical disfigurement (and even then I'm sure that there are some women who love it). Men cast their nets wider than women, whose taste is basically muscular and masculine or emo/scene type feminine. The latter is an example of how many people mistake females for being less shallow in this regard - When women say they like nerdy guys they mean men who wear chunky black framed plastic specs, the emo/scene/indie kind and who maybe own a macbook and a wii. In short, they aren't real nerds and fit into a very narrow dichotomy of what women want.
2) This is the key point: Women INFER more from good looks than men do. While it's true good looks trigger something in men that is quite animalistic and says "I want to have sex with this woman", we tend to leave it at that. We won't think a woman has a good character just because she looks good. Women are different - They will infer that because a man is good looking he is also charming, funny and intelligent right on the spot, and even if he doesn't fit the bill they'll convince themselves he does. The fallacy of claiming women go for funny men is a key example of this: Women do not go for 'funny men', that's a case of people misinterpreting causation - If you can get into her pants, she'll laugh more at your jokes because she's attracted to you. Effectively it wouldn't be exagerration to say women develop rigid opinions on someone's character from the moment they've laid eyes on them.
I think you've also got to add on the fact life tends to be easier for women in the 16-30 range and they tend to be able to date above their station in terms of looks a lot more easily than men do. So this kind of behaviour isn't really a problem from their perspective.