Units of the fort were only 4

I would like to know if there is a reason why only the Egyptians have their fort units unlocked in age 3, and the other civilizations can only use their fort units in age 4?
I did some tests and these age 4 units are not broken at all, they are just good in specific situations.

In the case of the Greeks, for example, they already get powerful units in the III Age. Units that counter infantry, cavalry, and archers. So, having the addition of their mythical troops in the III Age would be quite overpowered.

Atlanteans can build Destroyer, and Norse can build Huskarl and Portable Ram. Greeks can build Petrobolos.

Everyone gets something from their fortress-type building in Heroic Age.

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