on top of that all small planes have blind spots - high wing planes, you cannot see what is above and low winged what is below. as co-pilot my job was to peer around actively doing sky sweeps to spot traffic. add to that helicopters can fly into and out of zones very quickly.....even in traffic controlled areas there are pockets where you're not under tower control (air traffic control fans out in the shape of an upside down layered wedding cake.

in short, flying is like driving a car where the roads are 3 dimensional, you cannot see in all directions and objects are always in motion.