A DC/DC converter is more efficient than a voltage regulator because the method for controlling the output voltage is different.
A voltage regulator controls the output voltage by adjusting the driver FET’s ON resistance. In other words, the dropout voltage, which is the difference between the input voltage and the output voltage, in the driver FET decreases the voltage in the IC.
This driver FET voltage decrease results in loss, so the ideal voltage regulator efficiency is “VOUT/VIN x 100”.
On the other hand, a DC/DC converter controls the output voltage by adjusting the ON/OFF time of the driver FET.
The ON resistance is low while the driver FET is ON, so the conduction loss is small while current is flowing in the ON resistor and driver FET. Switching loss and other losses in addition to the conduction loss also occur, but even when including these the loss is less than that of the conduction loss, which makes a DC/DC converter more efficient than a voltage regulator.
The above is a general explanation, and the efficiency will differ depending on the power supply specifications, IC quiescent current, or the ON resistance of the DC/DC converter.
There might be times when the voltage regulator is more efficient depending on these conditions, so it is important to select the optimum product based on the usage conditions.