For one thing, the FAFSA is a royal pain in the ass and no one should have to do if they don't need to
Second, the FAFSA is a family effort and needs input from kids and parents which can be really problematic in broken families.
Third, it is really invasive and digs deep into your finances which would be a really unnecessary invasion of privacy for some.
Fourth, if you don't go to college or trade school it really doesn't do anything for you. Why hold a HS diploma from kids that don't have a path to a higher degree.
Fifth, it is also really pointless if you have too much or make too much money. Once you rise above the levels of getting assistance, it is really just a waste of time to just to be told you will get nothing.
Sixth, it isn't even the metric that all colleges use, some use a different program to assess college need.
Seventh, the FAFSA doesn't even tell you how much you are due. It just tells you how much your family is expected to be able to pay (which is always higher than people think is reasonable). It isn't until that number is processed through a college financial aid program that it actually means anything.
There has to be a better way to make sure kids get what they deserve than to punish everyone getting a HS diploma.