Because none of it was naturally dry land. Royal Palm Beach was primarily swamp and natural preserve until a large-scale drainage project made the ground buildable, and the village incorporated in 1959 once that work was done. The canals the village still maintains today trace back to that original engineering.
Engineered ground keeps behaving like engineered ground long after the drainage work is finished. Here the table runs shallow, so when a pressurized line under a slab lets go, the water rarely stays put. It spreads sideways through the fill before rising through whatever seam it can find, and that surfacing point is what shows up as a damp patch, not the actual break.
Responsibility for the water here splits three ways: the county handles water and sewer, the village runs its own stormwater utility, and two special taxing districts, Indian Trail Improvement and Lake Worth Drainage, pick up a share of the properties instead.