Because the markets dictate what choices they can make. Like it or not, college sports are the way they are, there are thousands of athletes that don't complain about it, in fact, they openly appreciate the opportunity.
They are free, and no one is hampering their choices. Maybe if none of those top tier athletes accepted the scholarships the market would have to change, but there are far more people who want to live that way then there are that don't.
BTW, $18,000 disposable income is a far cry from poverty. Poverty is based on discretionary income, and even that for an individual is $17,000. You are smart enough to know the difference between disposable and discretionary.