There is actually a very cool in game reason for Garuda to have a mostly human appearance.

**Spoilers for Heavensward Below**

In the latter parts of Heavensward, the heroes travel to a gigantic floating island created by the Allagans called Azys La. It was a legendary research base, and contains a great many chimeric creations. One of the dungeons here, the Fractal Continuum, features many. One of the most curious examples is the Ixallion , who appear to be winged Ixal. The lore on them claims that they were soldiers created to put down rebellions on the surface and that they had an unstable genetic structure. Curiously, this relates to the Ixal culture's belief in Atylan, the floating continent, the 'gods' they were warriors for, and the loss of their wings. The Lore Book posits that the commander of the Garuda Airborne Division was a beautiful female general. It stands to reason (albeit implied) that the Garuda we know is a mythologized form of this human commander.