I think perceptions here of symmetry is just wrong.
AoE2’s civilizations may have fewer bonuses and “differences”, but the bonuses tend to stronger define a certain gameplay aspect of that civilization.
AoE4 on the other hand, features bonuses that are hardly gamechanging. While +2 melee armour is nice for HRE’s Spearman, does that allow them to only spam that unit and win? The answer is no. In fact, nothing in the game is like that. Every civilization in AoE4 does the same Rock-paper-scissor gameplay, and every significant difference in terms of AoE4 civ design, does not change this fundamental moment-to-moment homogeneous experience.
One shining beacon in AoE2 is that Castle units tend to actually be strong.
Because Castle units do not exist and are therefore just another unit in a building, AoE4 cannot make them too strong, since they are available in basic production buildings. So units like Horse Archer (which is a civ specific unit) is just meh. Why would you design units to just be “meh”?
Identity should be one of the motivating factors for people to pick up a civilization. The vast majority of players do not pick a civilization just because it has favourable winrates. Most players pick a civilization like say, Mongols, because they have that Mongol imagery in their minds, and gameplay NEEDS to accommodate that. The problem is exposed when whatever makes a unit special, is either broken, or barely special. Think how Mongol’s core mechanic of being able to move buildings being an utterly broken mess of a mechanic.
Or alternatively, think of how Prelates are even considered a Unique Unit. How Elephants are useless.
AoE4 developers have designed civilizations with too much constraint if anything. What sense is there in designing civilization specific units to be so god awful that every civilization ends up spamming rock-paper-scissors anyway?
At that point, it doesn’t matter if you’re playing Mongols, English, Chinese. All you do is make Spearman, Archer, or Horsemen. If you’re lucky, you get a special version of those units for bonus points. And that, is only a recent development.
EDIT: Makes me wonder if Landmark-specific units should be a bigger part of the game. I recall one of the main reasons of liking Chinese is because their Dynasty units are worth making; of course coming at a heavy cost. But, it creates identity and makes the fact that you chose Chinese matter.