Because if a sports program brings in $100m of revenue and a school has 50k kids, that's only about $2,000 per student. Tack off the sports costs and you're looking at maybe $1,000 extra per student, but instead of offering discounts, they may 1) invest in their trust fund for future growth, 2) pay down their debts to reduce the lifetime interest payments and save money down the road, 3) Roll the money up into more scholarships or 4) invest in infrastructure. None of those things directly lower the immediate cost for other students, but add tangible value to students and the institution.