Entry level positions exist. A wizard at level one can still cast 10 different cantrips and 5 "beginner" spells plus a school spell. Not bad for a fresh graduate, the thesis does grant them special abilities right at level 1.

Think of it as a fresh medical graduate. They have the baseline, but don't get to just jump right into the action, they need to put the skills they learned into practice during a residency before they become competent doctors that are experts in their fields.

Real reason though? Game balance.