Because it is really a discount to the parents, not the student. It is understood that few 17 year olds have saved enough money to pay MIT's tuition of $85k/year for 4 years and parents are usually footing the bill.

Yes, students who's parents have money but choose not to spend it get a rough deal. You can make a pretty strong case that it is their parents screwing them over, not the school. The school doesn't owe a discount to prospective students.

In my opinion, you're reasoning about it incorrectly.

What if I said: the price is the same for everyone, but people with less access to money get proportionally more assistance paying that price?