I thought the exact same thing. The girls will be taught to keep themselves pure for their husbands and anyone is crazy to think men don't go for that.
Also they have seen the way their house runs and understand it's up to the men/husband to aspire to and pursue the careers that interest them and be the breadwinner and for the girls to aspire to support their husband as housewives.
If their money is limited they will see it's frivolous to educate the girls whose job they can learn from their mother and invest in the boy's futures since they will be heading households.
The idea that men should pursue their interests while women should be supportive housewives doesn't even seem to be a theory that the Duggars ascribe to - I believe the two oldest Duggar girls are looking into careers in nursing or nurse/midwifery, both of which would require college degrees and careers outside the home.
EVERY girl should be educated and have a career. It is ridiculous and even dangerously irresponsible for a girl to not have something she is good at that will bring in money from the real world. What happens if the husband dies? Then what? Not every girl is suited for housewifery and in variable economy, a part time job comes in handy.