Yeah but TCAS works inside each airplane. ATC (and ground operations) require coordinating across multiple types of aircraft, at airports across the world, with high precision AND humans in the loop (there are A LOT of edge cases).

This is a REALLY hard problem that the US cannot solve alone. It would require extensive global coordination.

Not insurmountable, but this is not something you can easily roll out piecemeal. If even a single aircraft lacks the compatible equipment you're back to the existing system.