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

"plot"

Though, if you want a realistic answer, it's easier to do things to friends because they aren't on guard. Like, if you're facing an MMA fighter you're not going to get sucker punched because you're redirecting then to try and hit you. My friend would have a much easier time sucker punching me because he hasn't ever tried to punch me.

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