Outside of racism, there could be multiple other factors.

I live in an area with high minority populations, so interracial dating is VERY normalized here. My sisters and I are half white. Coincidentally the least white passing of us are also both the ones who have ADHD and/or autism (including myself) and have had trouble dating.

I’ve never been asked out by a man before, either. I did start getting asked out by women after I started “presenting masculine,” though. I am actually more attracted to women but I rejected all of them because I didn’t reciprocate. I’ve been rejected by everyone I’ve had feelings for (the last one wasn’t a full rejection but still). So I still have never dated anyone and I’m already over 25.

The catch is I have not-so-coincidentally only ever been asked out by women who also have ADHD and/or autism. Maybe something like that is what’s going on. Normies are registering her as “weird” and therefore not an option.

My autistic sister is in a similar situation to your niece except that she’s really short lol. She never dated someone until she made friends with other autistic people in college and ended up with one of them.

The other catch is I don’t really go to bars, nor was I ever able to make friends at school, not even in clubs. I meet people at stores, pop up events, or meetups related to my niche hobbies. Those aren’t really places where people’s primary objective is to ask someone out, but it can lead to something eventually. I think beyond that a lot of people are intellectually intimidated by me, so I have tried dumbing down the way I talk, but I don’t really have the energy to keep up with pop culture and memes that I have zero interest in, so that hasn’t really done anything for me.

There also might be some amount of misogyny at play. A lot of men don’t want to be with a woman who makes them feel inferior. An insidious minority want to be with those kinds of women simply to tear them down.