But you can. Nothing stopped me from queuing into Experts all throughout Creator on Warrior and White Mage. I even brought them into Savage before either was BiS. In fact, my first A9S on WHM, I didn't have any accuracy melds. You can play every job. They just aren't geared as quickly as your main(s) will be.
Because the system's core intent is to keep incentivizing content. Allowing infinite progression means you will inevitably reach the point where you'll have obtained everything you fancy, thus have no further reason to queue. This is made significantly easier without restrictions and runs the risk players will spam content early on only to abandon it that much faster. To use myself as an example as I regularly play all roles. Say I wished to gear Samurai, Warrior and White Mage with only tomestone gear. The combined cost would be approximately 18,030. Doing absolute nothing but daily roulettes means I'll have all three more or less geared within the 4.0 patch cycle. Therefore, when 4.1 releases, I have no reason to bother with Ivalice outside a handful of times because I might find it fun. Likewise, I wouldn't run the new dungeon or any roulette nearly as often-- if at all. Basically, the devs have lost someone actively queuing. Considering the grind nature of many players, observed through Light farming, Verity and etc, I wouldn't be alone in abandoning queues.
Once you factor in Savage drops and the abundance of ways to acquire tomes outside roulettes, it only hastens the process. This doesn't even cover content which drops lower ilvl gear. Think about it. How many people do you see wearing the 300 dungeon gear over Verity? I see very few despite being on Balmung. I don't think it's much of a reach to argue people aren't going to spam Omega normal for a lesser reward.
Of course, you can dodge these restrictions by having multiple characters, but Square assumes very few players will bother. And most don't. Even if they did overhaul things, you will still be gated by some other means, be it RNG or some other timed progression because there is only so much the devs can develop over a three month span. WoW may allow you to farm at your leisure but the process is extremely RNG dependent. Somewhat ironically, WoW's systems are praised more here than on their reddit sub-forum. That certainly isn't to say FFXIV is better or couldn't borrow ideas, but does hint people may be looking at greener fields, so to speak.
I think a better solution is adding more content in general or possibly allowing us to customize set bonuses based on content we do. Eureka could be promising in that respect since they have hinted at possible relic armours. Yoshida also seemed to like the idea of achievements based on completing fights in certain ways or without xyz. I believe WoW does it. Have to dig for the interview MrHappy did with him, but it sounded interesting.
On the subject on Savage loot. I, actually, dislike how it's handled. The weekly restriction is fine, however losing out on a chest because you needed to sub someone plan sucks. It also enforces players to stick with their static. I'd prefer allowing up to say, two players who have cleaned to not affect loot drops. They can't roll, of course, but everyone else can. From three or more, you go to one chest and etc.