The Mk.I eyeball is actually incredibly horrible at spotting traffic. Once I started flying with TCAS it blew me away how much traffic there is out there. I'd watch the little blue thingers flying around the screen, and even knowing what directly to look in, at what distance and at what altitude I still couldn't find a lot of traffic out there.
The big sky theory probably does more for you than your eyeball does.
Yes, but these are two training planes in a training area that is known to be congested. That is much different than two jet converging at over 400kts each.