I think it is because of how the kick command works. A monk will always be bare handed in this game.
Kick does damage based on your attack when bare handed. It wouldn't make sense to calculate, say, a sword's damage rate into a kick
Barehanded damage is only useful on a Monk. It can be useful on other jobs, but you have to equip the barehanded ability for it to be useful.
You can only equip one ability at a time when using a job.
Therefore, the kick command would only be useful if you were a monk or freelancer. But the beauty of the job system is that it is designed around combining abilities with other jobs. It's less about combining jobs with the freelancer.
Plus adding a kick command would require another level for the monk.
This is all speculative of course, but I think it makes a lot of sense.