They pay or face contempt of court, but note that not all arrears are not a result of not obeying court orders. Nor is the totals of arrears owed the total of unpaid child support.
A man can be ordered to pay retroactive child support on a child he never knew existed. Further, 70% of arrears involve compounded monthly interest that has accumulated. Also, with 80% of all layoffs being men, many of whom pay child support, who do not have the money to hire attorneys to get a modification, thus resulting in accumulating arrears, which than accumulate compounded interest.