Life in unpredictable. Not everything can be explained. And not every question has to be answered for third parties, even parents.
May not want children.
May not be able to have children.
May not have met the right partner.
May have other priorities that get in the way.
May desperately want children but keep having miscarriages.
Since many of the possible explanations are extremely painful for the parties involved, the best position to remember is that it's none of your business.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).