1. Those are a very small portion of the total available jobs. Even if every 18 year old had the aptitude, there aren't enough of those jobs.
2. It's a LOT easier to get into all of those fields with a relevant degree. As noted elsewhere, a degree is a strong signal that you can perform at least some of the work.
Most companies that hire for those jobs in practice require a college degree to get to the interview stage.
Also: youth with nontraditional backgrounds (e.g. women, ethnic minorities) obviously begin the IT hiring process at a disadvantage. Having a 4-year degree can level the playing field with "traditional" applications possessing only a HS diploma.
