Why would it need to be? I don't claim to be an expert on Normans, Greeks, and Romans, but these are groups that are famous for their shield-wall tactics, where groups of men (ideally) advanced in lockstep with overlapping shields. This was not how warfare was carried out in Mesoamerica.

On a recent episode of the AskHistorians Podcast, I spoke with /u/iphikrates about the evolution of the Greek phalanx. I'll absolutely defer to him on this, but his descriptions of Archaic Greek warfare stuck me as something similar to what was seen in Mesoamerica during the Aztecs. Rather than a solid organized line of troops, the army would consist more of knots of elite soldiers with better equipment than the hoi polloi, backed up by lesser troops. While in Archaic Greece these elite soldiers would evolve into the hoplites of Classic Greece, in Mesoamerica these are the elites of the military orders with what Hassig, in Aztec Warfare, refers to as "shock weapons," deadly weaponry designed for up-close combat.

We unfortunately only have tangential references to the order of battle and tactics used by the Aztecs, but the aforemention Hassig has done enormous work on the subject. He proposes that a typical Late Postclassic battle would have opened with a barrage of arrows and sling bullets, followed by a phase wherein melee forces would rapidly advance, with the elite troops hurling atl-atl darts as they closed with the opposing force. The ranks of each force would then meet and it is here that we would see the loci of elite troops with macuahuitl and chimalli (shield), back by tepoztopilli (spear) wielding subordinates engaging with their like-opponents on the opposing side.

To sum up, an Aztec chimalli needed to fulfill 3 roles 1) provide protection from the initial barrage, 2) allow for the carrying and casting of atl-atl darts, and 3) be useful in close up dueling. The chimalli was certainly suited for dueling, but also allowed for its carrier to carry a number of darts for hurling during the advance. On the first point, one aspect of its design is the fringe along the lower border of the shield. Made of cloth, leather, and/or feathers, this acted as an additional flexible barrier to mitigating missile fire while not adding unduly to the weight of the shield.

The combination of a light shield which nonetheless added a fringe for additional protection while stile allowing for the wielder to attack at a distance with atl-atl darts is why we see a ubiquity of such equipment in the Postclassic. We can even see it in artwork and in ceremonial/artistic representations. We see a shield which could be wielded on one arm, but which nonetheless provided a protective fringe and allowed for carrying multiple darts in the same hand. The atl-atl itself would in the dominant hand for hurling, but either attached at the wrist with a strap or simply dropped as melee combat ensued. The 4 darts in both depictions are in line with Mesoamerican numerology which placed importance on that particular number.

The chimalli, to sum up, was like any other piece of military equipment: adapted to the challenges of the time and place.