TL;DR: I don't look good and a combination of being a natural sperg and having a very religious upbringing gave me some social problems.
Long version: Combination of things. Some major ones are my appearance (fat, unattractive) and I don't have hobbies that help socially. But those can be overcome. The biggest thing is that when I was a teenager I didn't really understand that I could just ask girls out and see what happens. I felt like anytime I would talk to a girl they must've known I wanted to go out with them so I never talked to anyone casually and as a friend. I didn't understand trying and failing would help make things easier, and would help me learn to talk to girls. I also suffered from some oneitis in part because of being raised very religiously. So I thought "trying" for more than one girl at a time was dishonest / a sin. It just gave me a lot of hangups about relationships in general, and I didn't help myself because I figured everything was God's will. It all combined to make me a sperg at my worst, and just someone too shy and awkward to really get with a girl at my best. I kind of finally sorted all of this out in my late 20s / early 30s but the dating pool shrinks so much after 30. And it's basically 0 if you don't want to raise someone else's kids.
Thanks for asking btw, I try to talk about this on here when I can in the hopes of helping someone else.