Garlean Inability to use magic.

[Kirika-]

All Garleans lack the "genetic" trait needed to manipulate aether, however pure blooded Garleans may "see" it with their third eye.
As they lacked aetherial manipulation, aka magic, they were at an extreme disadvantage to every other spoken, beast, or monster on the star, all of which can "use magic" to some degree.
That's why they went all techie after being displaced by all the spoken races and used that technology to subjugate ones who could use magic, and use them as cannon fodder in conquesting the rest of the star.

Fun Fact: Thancred Waters, even though he is a Hyru Midlander and a Scion, can't manipulate aether.

[Ransu]

A
Fun Fact: Thancred Waters, even though he is a Hyru Midlander and a Scion, can't manipulate aether.

Fairly certain he could initially. His loss of aether manipulation occurred after the events of Y'shtola teleporting them away during the post ARR events with them traversing through the lifestream.