Other mistake: The umbrella used to shelter Paris and Helen during the parade in Troy is obviously modern, as it includes metal spokes like modern umbrellas.
Continuity mistake: When Achilles is burning Patroclus' body, the moonlight reflects slightly off his arms. While a body is burned, a person with a torch is always beside the one who places the coins, so the firelight would've shined off Achilles' arms as well, and yet it doesn't, indicating that the scene was not shot continuously.
Continuity mistake: After the Trojans were ordered to push forward after the Greek's first attack, blood can be seen suddenly appearing on a few of the Greeks shields.
Factual error: When we see Agamemnon on his throne at Mycenae, there are ships virtually outside the door. Whilst the sea at Argos is just visible from the hill of Mycenae, the citadel is several miles inland.
Revealing mistake: When the soldiers of Greece are collecting the wood for the Trojan horse, you see bits coming of the wood, revealing the white Styrofoam used to make the wood.
Answer: They didn't know the Greeks weren't bringing cavalry. The Greeks did use them and sometimes brought them by sea. It was there to stop a cavalry charge and to break up fighting formations.
LorgSkyegon