Why modern warships cluster near 30 knots

Why do most modern warships still sail at roughly 30 knots?

Going faster becomes disproportionately expensive in power, machinery volume, and total hull size.

Higher speed requires disproportionately more horsepower as a displacement hull climbs the steep part of its resistance curve. The machinery then claims a larger share of the hull, forcing the ship to grow if it is still to carry useful weapons and sensors. That larger hull and power plant also raise procurement and operating costs, so a modest speed increase can consume resources better spent on combat capability. Sustained speeds much beyond the low-to-mid 30-knot range may push a modern warship toward nuclear power if the required output is to fit efficiently inside the hull.