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.

This is true on an individual level but not a macro level. Not everybody can be a developer or IT professional. The market just can't support that.

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.