They don’t fight back/use Haki to block like an opponent in an actual fight would.

In SBS he once explained that these hits between friends or family have a lot of love imbued in them, which renders things like Luffy’s rubber useless for defense. It doesn’t work on enemies because there’s no mutual love between Nami and her opponents.

That’s just an excuse though, the real answer is it’s cartoon logic

Edit: references: https://anime.stackexchange.com/questions/8001/how-can-namis-punches-hurt-luffy-so-much