But Darius gets Immortals no?
Yes, but the Immortal is a unique "normal" Chariot, it's not a +1 base-STR amazing War Chariot that is even able to beat Spears, it's a slightly stronger Chariot against Archers. The Chariot itself and the formula with which CIV calculates the battles is the problem, the 50% against Archers are deducted from the Archer and that one has an even lower base-STR of 3, so only 1.5 get deducted. The Archer has insane bonuses in cities and especially on Hills and Walls + Spears are even too much for Horse Archers, and as
kovacsflo once formulated: "The 3rd city of the AI at latest will capture Iron or Bronze" . The +1 base-STR of the War Chariot is massive, if you ever played Rome and had Praets you'll understand this. The Immortal has a very small time-window because once an AI has metal, that period ends, War Chariots however often have better odds than Immortals (even against Archers! ) because small differences in base-STR are huge differences because of the formula. I'd like to describe this like "against an Archer, even a normal Chariot can do given 2 against 1, an Immortal is only merely better. Against Spears and Walls however a normal Chariot is useless, the War Chariot, being better than a HA, may still succeed though and thereby has an era that can last until Pikes so Engineering (if adding Siege-support later) " . I know that this sounds a little weird, but a Longbow after 2-3 Catapults is beaten by a War Chariot because it's 5 against 3 in terms of base-STR (after Siege) , I don't know the odds by heart but I'd suspect that that'd be 70%+ odds, that's amazing given the cost of those units and the time at which one can have them. With all multipliers, Chariots may even get a 30-50% win chance after Catapults and Spears again would go down aswell, but exactly that is what an Immortal can't do because of the "little" difference of 1 base-STR. If this is not understandeable, see how normal Swords are crap-units against Axes while Praets get 80%+ , beat Longbows and last until Infantry with Cannons. Noone would think about doing Swords + Cannons against Rifles but with Praets that's possible, it's the same for the War Chariot, it exceeds the era of Spears and even can do against Longbows, it's not just 1 more STR, it's an era that is at least 3-5 times as long gamewise.
Sry if this is too off-topic and too confused

. I basically learned that the power of a unit comes by its base-STR because the formula weights small differences too large, because some bonuses that are written to be added towards the attacker unit are subtracted from the defender which changes a lot, and I have experienced that certain units greatly exceed their era because cost-efficiency is all that matters and because the upgrade-formula is a complete fail aswell.
[EDIT]
Try to see it like this: 5 against 6 is possible, 4 against 6 however isn't efficient anymore. It's simply that 5 against 6 is still 30-40%, 4 against 6 is already 10% and 3 against 6 is below 1%.