8 year olds go to school in the same town they live in and the school district has busses to go pick them up or their parents can go drop them off quickly. The same can’t be said for a student going to a college in a different town/city there’s not going to be a public transportation option that just goes straight to their school or to their job from school or whatever they need to do. They might be able to accomplish it by switching busses a couple times or something but that will add a ton of time to their commute. It’s not practical.
On top of that, pushing the driving age back will just result in a bunch of inexperienced 18/21 year old drivers and we’ll be having the same conversation about them after a few years go by.