Your advice is for an experienced driver who wants to become even better. The original commenter's scenario talked about novice vs. experienced. If you just tell a novice driver to scan everywhere, it's not actionable advice. Since, as was pointed out, they have not built a good filter yet. They'll end up being stressed out looking all over the place, when the simpler advice that driver's ed gives like "check these 3 places every 30 seconds", while far from perfect, will get them looking around a little bit and training that filter without becoming overwhelmed.