Why modern warships often stop near 30 knots

Why are many modern warships only around 30 knots?

Going faster is expensive in power and internal volume, and only front-line carrier escorts consistently need the extra speed.

High speed is not free capacity: machinery and fuel compete with weapons, sensors, endurance, and survivability. Costs and machinery volume rise sharply above roughly 30 knots. Carrier groups and other formations expected to fight at the front of a fast task force have the strongest reason to pay that price. Ships operating farther back can accept lower speed and devote the saved money and hull volume to capabilities more relevant to their role.