Because most people aren't that aware of their friends and relatives' hobbies and interests, nor the specifics about how they work. So if that person doesn't speak openly about them (and let's face it, most people won't simply because the other person won't find the topic that interesting), then they'll only know the most basic aspects.

Like, someone I'm friends with regularly goes to the climbing gym. I vaguely remember being told which gym it was at one point, and I know the sport they partake in, but that's basically it. I don't know if they've got any friends or acquaintances they hang out with each time they go, if they go to events related to it, whether they discuss it online, etc.

And the same goes with what most people (friends and parents included) know about me and my own hobbies and interests too. They might know some of the things I'm interested in and some of the people I'm friends with, but it's certainly not all of them.

In general, people tend not to know everything (or often, even that much in general) about their friends and loved ones. They know what they're willing to talk about/have bothered discussing, and that's usually it.