I mean the US does have debtor's prison. Fall behind in child support and you'll be thrown in jail, have your licenses revoked, property and vehicles seized, and passport confiscated. It won't be dischargeable in bankruptcy either. You then get to try and re-enter the labor market with a criminal record, and probably with even more arrears with looming further action. You basically become civilly dead.
IF you formerly had a very high wage job it can also be nearly impossible to convince a judge that your 'imputed income' should fall again, even if industry has shifted and you no longer can find those high wage jobs anymore. 20% of your pre-tax imputed income can easily become the majority or worse of your post-tax income if the bottom falls out of your industry, and then you are fucked.