Yeah it doesn't make a lot of sense. For every reason I can think of there are several counter points.
For ele vs monk you could make the "theme" argument when looking at the different elements and then when looking at monk just say cover df/healing/protection under one staff and wand but then separate the smite stuff into its own. Any secondary monks can use these off hands to pair with a main hand.
Inspiration is a very secondary attribute and its low breakpoints often support that. Also it is stacked with mantras which are stances.
These are weak debate points but nothing else really makes sense. Not even lazy makes sense since there are already a lot of skins in prophecies and some are fairly amazing.
A point against staves like ghostly staff not having ALL the attributes could be that it would water down the pool even more and in some cases unnecessarily.
Tl;dr
Monks use df with protection and healing prayers
Secondary monks just use their own caster staves or wands while onehanded martial weapon usesrs just take the available focuses in the offhand.
Inspiration magic is just not a build focused attribute and is presumed to be run along side another main attribute. Martial uses can still pair an offhand with a one handed weapon though.