Perhaps because the games theme was the colonization of the Americas lol. Its like saying a game about the American civil war doesn’t have European units
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They had way more unique units and mechanics than the predecessor AoE II. Also news flash, most European civs used Pikeman, Crossnowman, Musketeers etcetera. A Dutch pikeman might look different visually from a German pikeman, but its functionality was still the same, making all the civs have unique units with slightly different stats, first of all is impossible to have them all unique and secondly is impossible to balance and make it player friendly to remember all that shit.
The Natives had all unique units, as they didnt make use of pikeman so it was logical for them to change that, unlike AoE II where all cultures share the same basic units.
Before Asian Dynasties the time frame was 1492-~1890 with only the campaign of the Chinese making it earlier to like what? 1440?
Discovery age untill fortress age is roughly from 1492-1750, as the periods arent really historical before the industrial age, it is hard to determine the individual spans of time as the Age of Discovery spanned ftom 15th untill 17th century, which would coincide with the colonial age if you count that start date from the moment the Europeans began to intensify colonialism.
But so lets divide it up in roughly equal periods.
Discovery: 1492-1578
Colonial: 1578-1664
Fortress: 1664-1750
In the period 1578-1664, musketeers were most definatly used. So your assumption that it wasnt is just straight up false.
A unit that no one will eventually use as Janissaries perform the same anti cavalry role while being better? It also is yet another unit performing two roles in one breaking the balance of the rock-paper-sciccors system, a few units are okay, but there are way more then needed now.
As it would be a complete mess of balance, really hard to remember every single unit and completely unecessary as while the Ottomans have some claim to new units, the other European civs all shared the units historically so it is already historically accurate to have them have shared pikeman, halbediers, musketeers, you name it. Royale guard units are the ones that give it a little more flavor