Why not? Other jobs have taken directly from their main series counterparts, with MNK being one of the few jobs in the game bearing no resemblance to how it is in traditional FF games. I'd hardly say that it's defined too since SE wasted 6 years giving MNK absolutely nothing to play with in terms of visual identity. It looks exactly the same as it was in ARR, and you can't say the same for the other DPS. It is completely valid to like the visuals of MNK as it currently is, but job fantasy is important and right now it basically has none. Insisting that it stick to only CQC/martial arts makes it more Pugilist than Monk.
Yes, I am aware that at the end of the day, what matters is how well the job plays and no amount of Square Enix brand particle effects and pretty lights can make a job good just on their own. (They already tried that with Machinist and failed) But it also needs to visually evolve alongside new changes to the kit otherwise you get no feeling of progression at all. MNK never had a "wow factor" to draw people in, it was only able to maintain a player-base that barely outperforms BLU at all because of diehard mains that will stick with the job no matter the change.
Tangent over. The topic of MNK's visual identity or lack thereof should probably be its own thread.