Because it really wouldn't be cool to be unaccepted by them? Especially when you have no choice but to see them, every other time that you see your friend.

Then the friend probably thinks that you've been friends long enough, to not want to take it a step further by getting to be friends with each others' family members. This is always like the case with close friends. There may have been talking over the phone every now and then; probably when they call you and you're with your fam, you hand the phone over, they say hi and stuff like that...it happens most of the time, just because.