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.

Then these people probably should not go to college in first place and instead become plumbers or car mechanics.

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.