Why? Other weapons don't have the same paths as staves. Staves have several exceptions, even among elemental trial weapons.
Usually there's two elemental paths, one for Attributes (STR and Attack, AGI and Evasion, etc.) and one for additional effects (weakens attack, impairs evasion, etc.). Pet job weapons get an additional elemental weapon for pet effects (axe for BST, staff for SMN and hand-to-hand for PUP).
Now, staff gets two extra paths per element just for spellcasters. One is for magic damage the other for magic accuracy. So not only is it an exception to every other trial weapon anyway, but it has something else that sets them apart from all other trial weapons (shared with the avatar pet staves): the effects of the staves are mutually exclusive. Comparing this to combining the PDT and MDB sword or the Attack and Evasion dagger are completely groundless, because then you'd benefit from both swords/daggers at the same time. You'd get a weapon that's superior to the two before. For example, you could maintain high attack while keeping high evasion on.
That would not be the case for combining mage specific staves, because you could not use the seperate effects simultaneously. If you cast a water-elemental spell, you would only get a bonus from one of the staves. And you'd get the same bonus from the (hypothetical) combined staff. As such, it's not a better weapon, only a more convenient one. It wouldn't require you to gimp your inventory just to be efficient. And I don't think that your playstyle should ever be hindered by your inventory (which, sadly, is the case as of now).
I'd be all for combining the mage-specific staves, all damage, accuracy and avatar ones. Even if it was an exception, it would be a very welcome one.
However, I strongly disagree with the relic/mythic/empyrean idea. I'd want the staff to actually be obtainable.