I think the point bookworm is making is that one can easily miss seeing other aircraft. It's been proved time and time again that this is the case, however good your lookout is.
Girdler says it's not hard to spot other aircraft, but actually, it is. You don't know about the ones you don't see. Yet still, whenever something like this happens, people blame bad airmanship, not just with the individual, but generally!!!
Are standards going down? Are pilots looking inside more than in the past? Are glider pilots any better at all this...judging by how close gliders sometimes get to me, I'm certainly not at all sure about that last one. I see no clear evidence for any of the above, yet everyone talks as though it's a known fact.
The only known facts are that the mark 1 eyeball is imperfect, and the skies are getting more crowded......