It's not just transactional - i.e., 'we benefit so we pay'; don't let the reductionists frame the debate.
We also should do it because people deserve freedom and opportunity. It's the Land of Opportunity, not the land of rapacious capitalism; it's the Land of Hope and Dreams, not of self-dealing.
In aggregate, having an educated populace is hugely beneficial to society as a whole, which includes economic benefits.
This does not translate to commensurate economic benefits to all the individuals who get that education.
Our society benefits from these people having an education; therefore, our society should pay for it.