BLM/SMN Staff Weapon Skills limited without proper subs

[Harukusan]

For the longest time I've been curious as to why BLM and SMN were always restricted from using certain weapon skills with a staff without the use of a proper sub job. Relic, Mythic, and Empyrean weapons all focus primarily on BLM and SMN (and I guess SCH but I don't know much about that job). Relic staff: BLM/SMN only. BLM and SMN mythics are staves. Empyrean staff is BLM/SMN/SCH. Not the jobs that are required in order to use specific weapon skills: Earth Crusher, Sunburst, and Cataclysm. The first two are used for red triggers, and Cataclysm could be used by said solo BLM or SMN to build their Abyssean lights, or for use as other triggers in new battlefields, or simply because it's the most damaging weapon skill as a BLM without cruor/atma buffs. Even just for kicks or self satisfaction we are unable to perform these weapon skills, and I'm not even sure why. If Black Mages and Summoners were predetermined to use staves, I don't understand the logic behind this restriction. Care to explain? Is it so difficult to lift this?