One of the problems with student pilots, or inexperienced PPLs, and lookout is that whilst they may very well be looking out of the window, do they really know what they are looking for?
I remember quite distinctly that it took me ages to figure out where I ought to be looking at any given time, and just what other traffic looked like at varying distances and relative attitudes. I consider that I have excellent eyesight, and I had the double advantage in those early days of a top class RAF instructor and an aircraft where lookout was made easy by virtue of a large bubble canopy. However, I was constantly embarassed by my QFI spotting traffic long before me, some of which I never even saw (even his guide dog had sunglasses and a white stick).
Nowadays, I'm a survey pilot, and quite frankly a good lookout and situational awareness are two of the things that keep me alive on a daily basis. If I know, or believe, there is traffic in the vicinity, then I make sure I get an eyeball on the little devil regardless of where I am, and I make sure that they know exactly where I am too. There are enough fat, dumb and happy plonkers in the sky to kill me, and I don't trust other pilots any more than I trust other drivers. Just because they shouldn't do something is no guarantee that they won't.
Lookout and situational awareness are two of those things that take time to come to you. Yes you can try and teach it, but like any skill you only become really good at it with regular practice over a period of time. There are a great many pilots out there who simply don't have either the continuity or the time to have developed the skill fully (or fly a/c which limit their ability to conduct a good lookout), and you should conduct every flight with that in mind. Everybody is fallible and you'll only die once.