See and avoid is a remarkably over-rated talent on the part of most GA-pilots.
The only scientific research I've seen in this area reinforces the point quite markedly. It was commissioned after the Cerritos accident.
24 GA pilots flew Bonanzas on 45 min flights with an observer/safety-pilot in good VMC. They were briefed that the exercise was about single-pilot cockpit technique and asked from time to time to rate their workload. They were asked to call any traffic seen. A C421 conducted intercepts aiming for zero horizontal separation and 200 to 500 ft vertical separation.
The results: in 64 intercepts, the C421 was seen just 36 times, at an average range of 1 nautical mile.
(J W Andrews, Modeling of Air-to-Air Visual Acquisition, Lincoln Lab Journal Vol 2. no. 3 (1989))
Most of us have also had one or more scares in good VMC where we didn't see the other guy until the last minute.
I wonder how many went completely unseen?