I forgot to add...

I think the problem is that the AI is programmed to only flank with cavalry against Hoplites unless they are chasing another unit and they get very close.

I think the AI is also very heavily tilted toward flanking with the Cohorts, hence the aimless running around that they did, apparently trying to find a flank to grab hold of.

The cav were just absolutely petrified the entire time. They didn't budge. The AI simply didn't see an opening, and never tried to attack until I basically forced them.