Final fantasy 3 monk vs black belt

Better as in more damage. With the Shura Gloves' +20 STR, the Monk just deals more damage than the BB

Well, if we're going for job level 99 with Wifi items, the extra strength from the Shura gloves isn't going to make up for the 2 extra hits per arm the BB gets with their Wifi bonus item. The higher your character levels, the more the BB will end up ahead since the strength difference gets smaller and smaller, too. (going to 0 at level 97)

and its Retaliate ability is far superior than the Black Belt's, which is just awful

Retaliate does double damage, but there's only a 25% a physical attack will actually target you. Many bosses can act twice in a battle, so if you're lucky enough that they attack physically twice, all retaliate does is break even in damage expectancy, only unlike regular attacks it's completely unreliable. (i.e. you could use retaliate for 5 turns and never get hit) That leaves only the Iron Giant (and perhaps Cerberos) as a target where retaliate is (on average) better than just punching outright.
Charge, on the other hand, lets you abuse the fact that many bosses only use their nastiest attacks below a certain amount of HP. You can punch a boss a few times until they're right above the limit where they start using their nastier attacks, then charge twice and punch them out in one turn without ever having to worry about those attacks.

In other words: these are not the reasons you are looking for.