The reason why they don't want to implement "Thief" is because they want the WoL to only be associated with moral and upstanding dealings. That is why you don't get to play as a Thief but instead get to join the Rogue's Guild, which is comprised of former thieves. Even Black Mage and Summoner, whom both use forbidden arts tow the line but are given the pass as you are using the arts to prevent others from misusing them. Thieves by their very nature steal, it's eponymous to its name. Even if you're a "good" Thief, it still means you are stealing, which is criminal and not something they want the WoL to do for story reasons. We have "Thief" in the game as a job, and it's called Rogue. They historically had several skills associated with Thief, such as Sneak Attack and Trick Attack, Gust Slash, and Mug. Rogue even upgrades to Ninja much like how Thief upgrades to Ninja in FF1.


The Rogue equipment in the game even closely resembles the typical FF1 Thief archetype