Aircraft can already receive traffic and weather information over ADS-B. Larger or more modern aircraft even have onboard collision avoidance systems that supersede ATC when necessary and also carry weather radars themselves.

Additionally, there are human factors to consider. One commonly read-out piece of data is the local altimeter setting which ensures that all aircraft under the control of the same facility have a common reference - you generally don't set your altimeter to the area you're in, but rather to the setting of where you're going. Airplanes already automatically receive that data, but selecting the exact setting requires manual intervention for precisely this reason.

Add in the need to allow older airplanes to keep flying, the need to be robust to signal jamming, and the inherent safety issues with packing an airplane full of gas and hundreds of people, and it quickly becomes clear that full automation is inadvisable.