It is kind of weird (and is even referenced in a fan MST of Final Fantasy VI starring the FFVII cast), but in terms of the game, a possible explanation (other than "It is what it is.") could be that the armor has its own magic or something. The majority of weapons, armor and accessories in Final Fantasy are likely enchanted, as they abilities beyond their real life usage (i.e. protect against a specific element) so it could be that simply wearing the armor somehow "magically" protects you, though this is basically just a late justification rather than something I probably actually believe, like "Yeah... it could be that."

Also, as a confession, it never clicked for me when I was younger that it wasn't full armor for the characters' bodies - I don't think I knew what an "armlet" was at the time, so I didn't think of it as a bracelet, and may have just thought of it as another word for "armor". I may not have even thought of it, until seeing this thread, despite seeing that "armlet" was apparently meant to be translated as "bracelet" anyway.