Many different reasons, but for me these are my top ones:

1. It’s difficult to trust others completely. Even here on Quora, it can be easy to find sad relationship stories where one person manipulates the other or where one person goes into a relationship for the wrong reasons (e.g. infatuation, idealization, other motives like money, hoping to fix an internal problem through a relationship, etc.) Of course there are some good stories too, but it’s often the sad stories that stick to us and personal experience has led me to believe that it takes time to evaluate if someone can be truly trusted.
2. Some people don’t stay long enough for you to get to know them better before things become official. Often people can start dating without really knowing the other person and then drama ensues when they discover that they have more non-negotiable differences than they can take. Some people enter into a relationship hoping to boost their ego or have an outlet for their physical needs, rather than hoping to get to know the other person better, enjoy their company, and build new experiences with them.
3. There’s freedom in being single. You can make decisions and make mistakes and blame yourself only. It’s like that lyrics from Fifth Harmony’s Scared of Happy, you can really say “this is what I want.” You can crush on whoever you crush on without feeling bad about it despite not acting on it. You can solve your problems without being judged on how you choose to solve it. You can go shopping without people constantly hinting that they can’t wait to leave. You can plan a trip based on all the stops you want to make. Most things will be done on your own terms. And if you’re the type of person who’s always had to do things to please others in the past, being a single adult is liberating.
4. I value commitment and strong character. Some people nowadays can date without caring too much about how the other person in the relationship feels. As soon as their infatuation ends or as soon as they “get bored” of their partner they just leave. They focus only on the spark and often the sexual aspect of the relationship, and not much on commitment and communication. They can easily go on Tinder for another date or pour their feelings out on Instagram and quickly move on. It’s like consuming one thing and then moving on to the next. The commitment aspect of relationship isn’t as strong anymore and if ending up in a broken relationship is so easy then I’d rather avoid all together.
5. I’m unlovable. I like to sleep and daydream to fill the void of intimate relationships in my life - socially, this is not adaptive but for me it’s functional. I’m ugly no matter how much I try to smile and present myself with makeup, decent clothing, etc. I don’t have the bubbly, open personality that a lot of people, if not most, find understandably approachable. I’m not skinny and my stature, condition, and hopeless thinking make it more difficult to lose weight. Even if I lost excess weight, I would still look wide and would not have that confident and skinny body structure. Let me just clarify that I don’t blame people for not being attracted to me and I’m not angry at people for not finding me as approachable as others. If I were them, I would find me less approachable too. I know that looks are not everything, but I would be lying if I said looks didn’t matter.