We do not know that they did not create good infantry units! In the 19th and 20th century, scholars looked at Greek stories that Greek soldiers were much manlier than eastern soldiers, and decided that this must have been a technical problem: in their view, something was wrong with Persian spearmen but not Persian bowmen or Persian horsemen. If Greek writers told everyone who was listening that the Persian King hired tens of thousands of Greek soldiers, that must have been because Greek soldiers were better. Documents from Babylonia were reinterpreted in this light. If some Greek writers mentioned Persian hoplites, that was obviously a desperate attempt by Persian kings to copy their enemies. This fit in with colonial, racial theories about the timeless east whose people were suited for skirmishing or cavalry but not for standing up and fighting and supposedly lacked the creativity and dynamism of Europeans. (Sadly, some academics have revived these theories in recent times).

At the time this view seemed convincing, but we now know some things they had not considered:

- Persian infantry sometimes beat Greek infantry! At Marathon, Herodotus says that the Persians beat the Athenians in the center, broke through their line and chased them. A hundred years later, when a Greek coalition army lined up on the Aegean coast, some of the hoplites started to throw down their shields and disappear before a single arrow had been fired.

- Greek writers admit that many Persian troops were "hoplites" or armed like Greeks! In western Anatolia, Cyprus, and Egypt where Greek armies often fought, these kinds of troops were common, and had been long before the Persians came.

- after Herodotus, when Greek spearmen beat Persian spearmen, Greek archers usually beat Persian archers and Greek cavalry usually beat Persian cavalry. To most of the ancient writers, armies lead by their favourite Greek general are just better than anyone else's army, and this superiority belongs to the army and not just to one kind of soldier.

- most Greek writers say more about Persian cavalry than Persian infantry, but they also have more to say about Greek cavalry than Greek slingers and archers. This was a matter of class: Greek aristocrats wanted to be like Persian aristocrats, and not like the farmers and craft workers who fought with bows, and it is wealthy Greeks who wrote and preserved most of our sources.

- there was not a decline in the Persian system of conscription. Tablets from Babylonia show that the Persians conscripted men or collected taxes to hire soldiers from all classes and nations. The things which earlier scholars saw as signs of decline in later Persian times are actually recorded in texts from the age of Hammurabi, so they are better understood as part of the system then as signs of decline over time.

- many of the technical differences which modern people see don't match the ancient sources. Some modern people imagine that only Greeks could march in step in regular formations, but Xenophon describes how king Artaxerxes' infantry marched into battle silently in beautiful rectangles, all slowly with an equal step (Xenophon, Anabasis, 1.8) while most Greek soldiers did not practice moving together in time at all. Herodotus talks about how Persian infantry were not as well protected as Greek infantry and had smaller spears, but he mostly seems to mean that they had different shields, and artwork and surviving weapons show Greek spears and Persian spears with the same range of lengths. Peter Krentz and Roel Konijnendijk (Iphicrates) have published articles on this.

So in the end, we don't know why some Greek infantry beat some Achaemenid infantry. Herodotus seems to say that it is only natural that people living the good life as rulers of an empire become less tough than their poor ancestors, but most Persian soldiers were small property owners, salaried professionals, and temple serfs not decadent aristocrats.