Simplest and easiest answer, money.

A lot of MMOs charge for extra characters. Some give you 3 free slots or so, but you have to purchase additional character creation rights and pay for it monthly. FFXI is a prime example of this, albeit you were not hindered on jobs, more on storage.. I believe Ragnarok Online as P2P only had 3 slots as well, but could purchase additional slots up to 6? It's been a long time, Thief->Assassin was most popular then. It was before they added a lot of other classes.

At any rate, if you're like me and play a majority of the jobs just so you have at least basic skills with the class, then you should be able to see how much extra money you would spend just to level an extra job or two, let alone another 15. ARR is kind of exempt from this in a way. Youll understand if you review ARR service fees posted a while back during the legacy campaign posts.

so let's calculate that.

12.95 basic service fee (includes 1 char)
1.00 Additional Char slots (max 32 slots, including the one with your basic service fee)

so, lets say we have 28 different jobs (not counting crafts).

that's $27 extra a month just for characters, if you play all jobs. Thats $324 dollars a year you're paying, just to use every job that is in the game to begin with.

With ARR almost exempt from this, we can still look at money sadly.

If they locked each character to 1 job (or class with underlying jobs as seen in FFXIV(Gladiator -> Paladin)), this would mean you'd be forced to purchase the more expensive option to have all 8 character slots per world, just to check out a job and decide if you like it. Not very rational. Now think about that. 8 slots per world. As crafting classes are considered to be main classes in FFXIV (atleast in 1.0 they were), this would mean you still wouldn't be able to play all the jobs available. And should they increase the number the of character slots we have, we'll be back to square one, paying for extra slots.

It reduces the consumers hemorrhaging of money on the game itself after monthly fees, making it a much more plausible to harness long time subscribers.

Of course, this is just my opinion but, facts are in there as well. It's just a matter of perception.