Why is "not having kids" treated as a default deviation that needs a tragic or selfish excuse, rather than just a neutral life choice?

Whenever the topic of not having children comes up, I've noticed a recurring pattern in how people react. If you say you don't want kids, the response from pronatalist corners is rarely, "Cool, that's a valid preference." Instead, it's often met with this thinly veiled pity or defensive justification, like: "Oh, I understand. If I had financial struggles, or childhood trauma, or if I just wanted to be completely selfish and prioritize my career, I wouldn't want kids either. I don't judge you." It feels like society operates on the assumption that having children is the absolute default, and not having them means you must be broken, traumatized, or morally deficient in some way. Why is it so hard for people to accept that someone might just genuinely prefer a child-free life without needing a dramatic backstory or a character flaw to explain it?