$30 / hr + federal payroll taxes is 5,700 / month ($30 / hr x 40 hrs/week x 4.33 weeks in month x 1.1 for federal payroll taxes). Who has this kind of money on top of mortgage, car payments, food, utilities, etc...? In my circle of friends only one family affords this (the dad is a Director at Meta)

The trick is that au pairs are nowhere close to that expensive. (Though the US becoming radically less attractive of a place for foreigners to live and work may be changing the availability of that option.)