A few reasons.
1. I dislike a lot of the world and don’t want to bring another life into it.
2. There are plenty of babies that need mothers, if I really want one I can adopt one.
3. I am still a child myself, and enjoy the freedoms afforded to me by having no children.
4. I don’t like screaming flailing shit machines.
5. My current long term partner does not want children. This hasn’t influenced me, but it’s worthwhile to note that we have the same general outlook.
I know I would be a wonderful mother, and my genetics aren’t seriously flawed so there’s no problems there. I just value my life the way it is now. I don’t want to be responsible for another person. I don’t want to be tied down for a twenty year and hundreds of thousands of dollars of commitment (a similar reason why I rent vs own).
And sadly, knowing all of this doesn’t damper my womanly instincts to coo at babies and think about being a mom. Hormones are bitches like that. I plan to fill any of these needs by being around other babies, babysitting, making cute baby quilts, etc. I’ll be the best Aunt ever, since I don’t have children of my own to spoil :)
I remain open to the option of having children later on in life, maybe, possibly with the right person. But all signs are pointing to no right now.
Edit: And I’m in my 20s.