Costs-benefits analysis. If I allocate $100 for 100 students in higher education, I can spend only $1 on each. If 25 show far more promise in the productivity they can contribute that $4 spent on each will achieve a greater overall output of productivity, it's in society's interest to invest $4 each in those 25 students over investing $1 each in 100. Since those 25 are getting $4 of investment, they should also expect to pay back to society enough to justify the higher investment in them.