Because now we’re talking about putting deeply integrated equipment in every plane. It’s a certification and cost nightmare.
This is not a system where you get to do clean slate greenfield development. Whatever you do must work for the lowest common denominator. ATC is a fairly cheap societal expense compared to developing, certifying, installing, and maintaining systems with the level of integration you want in hundreds of thousands of diverse planes.
The US has about 200,000 general aviation planes. You can’t ignore them, and you can’t just ban GA because that’s your pipeline for getting commercial pilots.
This is not a system where you get to do clean slate greenfield development. Whatever you do must work for the lowest common denominator. ATC is a fairly cheap societal expense compared to developing, certifying, installing, and maintaining systems with the level of integration you want in hundreds of thousands of diverse planes.