If you can't trust a professor to professionally and impartially grade someone's work, the system would likely collapse. This is not to say that there hasn't be

If you can't trust a professor to professionally and impartially grade someone's work, the system would likely collapse. This is not to say that there hasn't been cases where professors have been shown to be biased, there has. But the premise of universities is to give professors some autonomy in the way they teach and evaluate students.