Armor and helmets of roman commanders

[Flavivs Aetivs]

It's because of course of art. Items like the Ludovisi Sarcophagus, etc., which depict such equipment. Whether it actually existed is up for debate.

Early commanders may have indeed dressed in the Greek style. Late Roman commanders are usually mentioned as wearing gilded scale in the text (see Sidonius, Claudian, etc.).