immersion is likely the excuse the developers use to cover up some more technical restriction. It's not unlike how they told us at first they didn't add the opposite sexes for viera and hrothgar becuase of lore, but then later confirmed it was because they didn't have the resources to add all four and only more recently have they managed to rework their processes to allow them to squeeze them in.

The fact that glamor is restricted to items you can currently equip on the job is likely connected to some code restriction that keeps them from expanding things out further, and unless they made all gear equippible by all classes, they may not be able to work around it.