It becomes challenging then trying to establish a level of tax that brings in enough money to run the country, and doesn't absolutely hammer low income earners. If you earn very little money, a flat $20,000 tax is a lot of money and seriously effects your day to day life. If you earn a lot of money, a flat $20,000 tax doesn't change a thing. Makes sense to tax high earners more and low earners less to try to maximize everybody's quality of life.
I do agree more help for students would be useful. It bugs the hell out of me that I study a stupid number of hours each week, and when I head off to work at 7:30am on a saturday, I can hear my neighbors in newly built government flats, sitting on their balcony finishing up their night of drinking, discussing the merits of choosing not to work.