If they can't find them, how do they know they are "mistakenly defaulted" into the system?
Your quote is out of context. What the writer said was: "Because of the indifference of both the states’ child support enforcement systems and their federal funders, no firm figures exist on how many men have been mistakenly defaulted into fatherhood. Henry estimates that the number could exceed one million."
This is not the same thing as being "mistakenly defaulted in the system" (which the writer did not even state) because they could not be found. Additionally, the underlying problem with serving the papers, according to the article, is not so much that they "could not be found", but "Moreover, substitute service, where the court summons is often left at an erroneous last known address, is frequently used instead of personal service."