“Answer to the OP:
Because they’re too busy doing what they’re supposed to be doing.”
If I may engage in some pilpul over here:
R’ Yisrael Salanter (in Iggeres 27) writes that there are two parts to learning Torah – limud hatorah and yedias hatorah. Limud Hatorah is simply to learn any part of Torah whenever possible. Yedias Hatorah is that you have to know Torah which includes knowing the halachic conclusions and mastering the methodology by which they are reached. Seemingly, a woman’s chiyuv of yedias hatorah in topics that apply to women should be identical to a man’s chiyuv. They both have to know how to properly observe the Torah. Now R’ Salanter writes that one can be mevatel limud hatorah in order to further the goal of yedias hatorah. So a woman should also be able to be mevatel her other responsibilities in order to further the goal of yedias hatorah. If you say that she doesn’t need to know because she can always ask someone, you can say the same thing about a man.
However, R’ Salanter explains that the chiyuv of yedias hatorah is based on “V’shinantum L’vanecha”. Therefore it is possible that we we would darshen ???? ??? ??? (the Gemara only makes this derasha by ?????? ???? ?? ?????), and it would come out that a woman doesn’t have a chiyuv to know any Torah – she just has to be able to observe it properly and knowing Torah would be a hechsher mitzvah so to speak, whereas a man aould have a specific chiyuv to know Torah independent of enabling him to observe it. And therefore the chiyuv to know Torah could override a man’s chiyuv to learn Torah but a woman wouldn’t have such a chiyuv to override her other responsibilities. (It could also be that limud hatorah is an exception in that any mitzvah can be docheh it, but than again I don’t think that the responsibilities that a woman would be “neglecting” by pursuing advanced learning would be an actual “chiyuv” in the first place.)