Why did the Persian commanders drive their troops with whips?

The ancient Greek historian Herodotus wrote that during the invasion of Greece by Xerxes, his commanders drove the army with whips. True, the same Herodotus claimed that Xerxes led an army of one million soldiers. Most likely, there were 200 thousand of them.

Xerxes did not have a professional and hired trained army, and most of his soldiers were representatives of the captured peoples. Naturally, they did not want to die and kill for the sake of a foreign king, for a foreign people. They had no interest in fighting the Greeks.

At the Battle of Thermopylae, the troops of Xerxes were demoralized, as they faced a seriously trained army of Spartans and lost a large number of their soldiers.