If they did something like that, people will complain that they're forced to level multiple classes to level 50, and the guy with all jobs to level 50 is going to complain that it takes too long for him to hit his cap.

The crux of the problem is that this game is rather unfriendly for alts. The payment structure doesn't encourage you to do this, and just thinking about doing normal primals again is probably enough to discourage you from doing it.