This is one of those cases where parity sort of applies. If we set classes to start at lv1, DRK AST and MCH set the starting point for jobs at lv30 (which makes sense seeing that it's the point where you have access to job quests). Considering that getting a job at lv30 with a bunch of skills learned already stretches suspension of disbelief, having a job start at lv50 with 23 abilities already learned is just silly.
Aside from that, you have the learning curve. It is in fact easier to learn a job as you level it, and considering that the number of skills gained at 30 are already considerable, lv30 is a decent enough starting point where a job has plenty of skills without being overwhelming (which I fear would be the case with a lv50 job).
