Because the city itself was assembled rather than built from scratch. Miami Gardens incorporated in 2003 by joining seven previously unincorporated communities into one municipality, so the new city is really established neighborhoods under a new name. Most of that housing dates to the late 1960s, when Interstate 95 arrived and discriminatory housing covenants were struck down, drawing African American and West Indian American families of middle and upper income north out of Miami.
That timing carries a practical consequence. A house from the late 1960s sits on the cast iron side of South Florida’s drain-material line, and cast iron rated for forty to fifty years frequently starts failing here at twenty-five to thirty. The city turns twenty-three this year; the pipe underneath most of it is pushing sixty.
The other live question in this part of the county is septic. Plenty of properties in northern Miami-Dade are still on it, and the county publishes a formal route for switching over to sewer.