Before someone says, pick and choose whatever item that goes to the job, think of how many ToM weapons that certain jobs have; now add that together with 20 jobs and suddenly the extra space we have is no longer there.
Plus, we are unable to store or transfer ToM weapons, augmented items or certain items.
There's been a large introduction of items considering the last item bag update, with most of them being able to be sent to the same account. However, even then, people would require a large variety of items to place themselves at their best (tp/ws/nuke/rest/idle gear swap) which would mean you would have to carry that gear with you at all times.
And lets face it, there's always new items coming out with each update or so, and only few of them are transferable or can be stored.
For those that don't seem to have that problem, its not a matter of being the best, but its a matter of having the space to hold items, should that need arise.