Because it is really hard to understand something that isn't your experience. Coming from the other end, I had a hard time understanding sexual attraction was an actual thing (being ace and growing up very religiously), and it took a friend explaining to me how it was real and some time thinking about it before I accepted that.
So it just takes some time. There's a difference between people who just don't understand, and people who refuse to listen or put in the effort to understand (and idk which one your cousin is but I'm just saying)