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Mostly because it would be a pain in the rear to have two separate progression lines for new jobs.
They need to keep the "start from level X" progression in the game, because veteran players for the most part don't want to have to start at level 1 for expansion jobs.
So if they added them to character creation, they would no only have to have a separate progression system for starting them from level 1, but they would also have to create like 20+ new job quests to fill in the 50-60 level gap. That's a ridiculous amount of work.
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HW jobs are in ishgard and would take too much effort to move elsewhere.
The others start at too high of a level.