The problem isn't collision detection or predicting movement. They're not a bunch of particles on simple ballistic trajectories. They're powered objects traveling in a turbulent and difficult to predict medium. In emergency conditions they can turn from a powered vehicle to an unpowered one. They can need to land immediately when flight worthiness changes in flight. A situation on the ground can make landings unsafe or impossible and an aircraft needs to diverted disrupting traffic at another airport.
Automating ATC works until one or more exceptional conditions arises. Then it's completely unsuitable and everyone from pilots to ATC need to work against the happy path automation to keep people alive.