Because they want to receive the full $80k from the grant - often federal - and pay the student $25-30k. The right answer is to fix the accounting measures. For a PhD student primarily doing research which benefits the university and grantor, and taking no or few classes, why are they paying tuition? If it’s for facilities or university staff salaries, then just send the full grant to the university directly, and let the university pay the students whatever both parties see as fair.