Because that's never been the point of their character? Because Nami herself is pretty non-combatant and prefers not to fight? Because Robin, despite having ability, has never been shown to care about fighting, and only did so when it was for something else? I don't think it's hard to understand why both Nami and Robin don't get lot of fighting.
But by all means point out the obvious important plot-reasons why they should fight and why it just isn't fan-fictions of people that fail to understand the characterization of these characters.
This argument really reminds me of people who were whining about Rebecca not fighting despite thematically she was in character, and was only taught to defend herself, and it wouldn't make much sense in her fighting when it goes against something that was already established chapters before.
So why do you single them out from the rest of the crew.Because theyre female? I dont see how they are different from the guys aside from luffy and zoro nobody really likes fighting but they do anyways because they have to. They are not like Rebecca where it was key part of her character to not fight