I spent many years not wanting children. I got asked why constantly- like literally from the time I could have children (in middle school) to like 6 years ago when I finally did have kids.Why is it difficult for some parents to understand that some people simply don't want any children?
It is the natural drive of humans. Society, playing the violin, reading a book, drawing a picture- are all things we do in between the basic and primary functions of our biology. People are not wrong in saying we are just existing to make more of ourselves; not that life is that narrow, but it is that basic.
I had my kids pretty late in life. Turned out I really enjoy being a mother in ways people told me I would (so a lot of people were right actually!) and in ways I didn't expect. When the urge to pump out the babies came it overwhelmed me like a desire that I can not properly articulate- it was like severe depression and restlessness to get pregnant. I would see babies and get excited, wistful, depressed, happy, etc. Even cartoon babies triggered tons of emotions.
Perhaps that urge doesn't overtake one in their life. That is fine, valid and sensible to not force on yourself. However, having been on the other side of the 'no kid' fence then waking up one day and feeling like I wanted to have 8 children in a row (no joke)- it's a very, very strong desire that deepens over time and the length of a relationship.