It may be that visually equipping an item is inherently locked to the coding that checks whether you can wear it as your current class, or it may be an aesthetic choice by the devs.

My impression is that they are happy to share some designs around between classes but others they want to keep locked – job gear in particular, but also some sets of dungeon gear get assigned to the same roles every time they are reused, and those are usually the more distinctly role-aspected ones with mages in robes and tanks in armour.

I suspect that if it is possible to unlock glamours, it could be "all or nothing" and they don't want unlock all.