$6,000 / month in daycare for 4 kids? You have a sweet deal my friend. At the daycare in my neighborhood this does not cover even 2 kids : https://www.kidspaceseattle.org/enrollment - click on Tuition link at the bottom and weep.
> Why would somebody ever pay that much rather than just hire a private nanny?
In my experience the price distribution for nannies and infant/toddler daycares in the same area are largely overlapping bell curves so the decision typically comes down to logistics rather than cost.
Also, a nanny is not the same ecosystem as child-care. It's not nescessarily a substitute. If what's desired is a private-private child-care, I suspect you'll be paying a lot more than the price of a nanny.
One challenge with hiring a nanny is if they need to take a sick day (or if they quit!) you can end up in a tough spot. In contrast a day care center usually has backups built in so you don’t end up scrambling.
$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.)