In free flight, two aircraft can be anywhere in 3D space and collisions are rare. Around takeoff and landing - there is effectively a 1D track for the approach

In free flight, two aircraft can be anywhere in 3D space and collisions are rare. Around takeoff and landing - there is effectively a 1D track for the approach and the climb-out. It's not quite that simple because different aircraft will have different descent profiles and different angles to the wind, but it's a much smaller 'search space' for collisions.