Probably because you'd have to
A) Be a psyker who escaped the Black Ships in the first place, which means being either very weak or very lucky.
B) Survive to adulthood without losing control of your powers and blowing your own head off or being eaten by a demon, bereft of the training the Scholastica Psykana provides.
C) Be willing to forge your credentials of Scholastica approval in the risk (small, but existent) risk of horrible, awful and fatal punishment if caught.
D) Find an employer unlikely to check those credentials.