I'm 29, and there are a lot of reasons.
Primarily, I've just never had a successful relationship. I've never had anyone who actually liked me back.
I've asked people out. I've gone on a few dates. Usually, I either get completely shut down, or they break it off after a couple dates.
I know it's probably because I have a lot of issues opening up to people. I'm a painfully-uptight person until I get comfortable. I'm trying to be more comfortable with myself, but I've just had so many relationships (not just sexual) where opening up to other people got a negative response. When someone asks personal questions, I just reflexively stonewall. Redirect the question, answer vaguely, and sometimes lie to maintain the image that I want to project. In fact, this is an old burner account, because I don't want this post in my post history, in case one of my friends sees it. I try to be approachable and friendly, but I'm sure every fiber of my being is shouting "I'm uncomfortable, please go away."
At this point in my life, I don't even know if I want a romantic relationship, or if I could even handle one. TBH, I'd be happy with a larger circle of friends. I've got a small circle of friends, but we're scattered across the state. I get to see one every couple of weeks.
I've forced myself to leave the house, to try and make local friends, but I can't open up. And when I try to approach people, I just think, "they don't care about what you're going to say, they don't want to hear from you, you should just stop talking and go away."
I just know that I make other people uncomfortable, and I don't know how to fix it. At this point, I'd be happy just having a small circle of friends to go to the movies with, or meet after work. I work alone, I come home to an empty apartment.
I can't even keep a cat because of my allergies (which have ended more than one relationship, all by themselves). I want to tell people, "if you want me to come around more often, the cat has to go," but I can't ask someone to do that. My own parents picked up a couple dogs as soon as I left for college. I've seen how happy owning pets makes people. I just really wish I could make someone that happy.
Sorry, I got way off-topic. I don't get to vent much.