Owner's manuals published in the '90s and 2000s copied brake-fluid intervals from vehicles designed in northern climates. Florida is not a northern climate.
DOT 3 and DOT 4 brake fluids are hygroscopic — they absorb water out of the air, even through the hoses. Water lowers the boiling point (a fresh DOT 4 boils around 500°F; contaminated fluid can drop to 280°F) and corrodes ABS modules from the inside.
We measure moisture content with a refractometer at every visit. Above 2%, we recommend a flush. Above 3%, we insist. For most Duval County commuters that works out to every 24 to 30 months, not the 36-month book number.