Because those daughters are busy fulfilling the responsibilities of their husbands, children, and in laws (the last ones not being farz but ehsaan). That happens in majority of the cases unfortunately so they hardly get time to look after their parents. Still, they try to do whatever they can in their capacities. I've seen working married daughters supporting their parents when they have no such financial responsibility either. Irony how easy it is to blame women in our society who are already oppressed and do shit loads of things they aren't supposed to. Hmare mazhab may to women have full rights to have full time house helps and caretakers for their own chores, still they end up being unpaid, unacknowledged servants yet no one talks about that religious aspect.