I assume it's something to do with spaghetti coding, and how classes and jobs are programmed to be seen as "separate classes" rather than one being an extension of the other. (For example: not being able to equip/remove your soulstone mid-dungeon because you're not allowed to "change class" from the one you queued with; also not being able to complete conjurer's unicorn mount quest as a white mage.)
It may not be possible to bar classes from selected duties while allowing them to access others. Either that or they're clinging to the "classes are equally viable as jobs" mindset, but everything else suggests they abandoned that approach long ago.