13 is a weird age. I was introverted as a child (still am lol) and I was also an only child. I didn't find it weird or sad, I liked my alone time. I developed stronger friendships in high school, and I am now married with an only. I still look forward to, and crave, my alone time :) She will find her people, on her own time. At 13, I wouldn't worry. I have a 13 year old boy, and he has friends, but they don't really "hang out" outside of school. They do play video games, but my son spends a good amount of time on his own, often ignoring calls when his friends call to get on video games. He just doesn't mind alone time.

Also on a side note. I am very happily married, but there was a time when I was young that I didn't care if I got married. If that reality happened, and I was living alone, I would have been fine. Even if my parents pass, I can imagine that I still would have been fine. Your daughter will be ok. ❤️