There’s a difference between childless by choice and childlessness because of natural infertility, etc. presumably the parents didn’t choose their children to be burdens and so we’d chalk that up to statistical probability.
How do you know they didn't? Some people don't want to screen for birth defects or check for compatibility between themselves. They don't do all preventative measures, don't use gene-editing therapies or do harmful things like drinking and smoking while being pregnant.