In reality, no ancient army (except a HA army) would attack a phalanx-army that was on top of a hill. A phalanx was very hard to penetrate frontally on level ground. Trying to do it while struggling uphill would be suicidal, unless the attackers had a massive numerical advantage.

So the AI is doing the right thing by not attacking under such unfavourable circumstances. It is better for them to wait at the bottom of the hill and force you to attack them.