Not to be rude, but its designed this way. Atma farming, dynamis, nzule isle was a grind. Its designed for end-game grining. If players could clear things like this in 1 month it would leave players dropping subs. Prime example is SWTOR.

Where its not required to "have" these weapons its designed to make you work for them. Making it possible to obtain the item while on another job/class is pointless. You may say well I can level another class while doing this for X class takes away from extented playtime and content. FFXI did something like this when FC parties where formed. It did not add time to the game it lowered time to the game.