It was a pain to balance, both for the devs and the players. Ask any player who mained both how they loved having to respec or have a weaker spec because they didn't want to have to waste GC seals to grab the respec item every time they went into a party for something. Spoilers: we didn't. It was an interesting idea, but the devs split them enough eventually; they're still intertwined with abilities that are shared, but it's no longer "I want to play SCH but all my stat points were allocated into INT and not MND, gdit I have to get a Hymn now" to be able to play healer over dps, or having your stat points allocated evenly and being weaker in dps or healing because of it.
One is a caster, the other is melee. It was one of the last things Yoshida commented on during the keynote.
Overall, I do agree with others here: you can queue into Sastasha or a potd to get accustomed to a job enough to use it. Generally, dps isn't too hard to figure out if you look at your tooltips: is this part of a combo? Do you use it in conjunction with something else? You don't need to be a perfect pink parse or whatever colour it is now on day 1, especially in casual content (note, I am not saying you shouldn't be trying to do well, here: don't be a freestyle SAM, but you also don't need to go hard on having a perfect rotation in the first week of the xpac while you're learning the job). It's the same with tanks and healers; if you're good at proactive healing on SGE and SCH, you'll pick up whatever new proactive healer they release next. If you're great on tanks and have played them all for a while, chances are you'll pick up whatever tank they release next fairly quick since there's plenty of similarities across the board.
Most want to be able to run the xpac with a new job, especially if it's something they're willing to give up their old job for to main. Having it start 50 levels under the max of the xpac would turn off a lot of paying players. As it stands, you can't unlock the new classes from ShB or higher on the free trial: they're locked to the xpacs, not levels. If a trial player wants to play a class, they will either wait for the trial to be expanded or become a subscriber.
I wouldn't mind if they did something in the intro quests as you unlock jobs beyond what they have done, especially recently, with the quests leading up to old xpac max level, like a couple of intro quests synced down to get you into it easier, but it doesn't need to be starting lower levels.