AoE3 already had 16 years to build its roster of civilizations. It currently has 22 main civs + 24 different revolutions playable, each with its own fair share of unique units and mechanics. AoE3 doesn’t need filler civilizations to make its gameplay more varied.
AoE4 released two years ago, it currently only has 10 civilizations to pick from and it will have 12 with the new expansion. Variants civs are a clunky way to add more variety to the game quickly for a cheaper cost.
The two games are in very different situations.
This whole situation makes me appreciate AoE3’s design choices even more, stuff like the customizable decks, native allies and revolutions create so many different possibilities that are simply not possible in AoE2 and AoE4.