A lot (most?) jobs don't even really require finishing High School, TBH.
But keeping people in school longer and longer has a couple of effects that TPTB find helpful:
more time for leftist indoctrination
job creation program for teachers
delays marriage therefore reduces birthrate
reduces the time you have to develop self-reliance skills, meaning you will be dependent on the State for things (even above the indoctrination factor).
pure academic types are less likely to start businesses; keeps the competition down for the multi-national corporations
A lot (most?) jobs don't even really require finishing High School, TBH.
But keeping people in school longer and longer has a couple of effects that TPTB find helpful: